diff --git a/docs/de/USAGE.md b/docs/de/USAGE.md
index 5416139..2bfb008 100644
--- a/docs/de/USAGE.md
+++ b/docs/de/USAGE.md
@@ -24,6 +24,7 @@ einen Rundgang oder springen Sie direkt zum benötigten Feature.
 - [SVG](#svg)
 - [Hyperlinks und Lesezeichen](#hyperlinks-und-lesezeichen)
 - [Formulare (AcroForm)](#formulare-acroform)
+- [Ein bestehendes Formular ausfüllen (AcroForm)](#ein-bestehendes-formular-ausfüllen-acroform)
 - [Annotationen](#annotationen)
 - [Verschlüsselung](#verschlüsselung)
 - [Digitale Signatur](#digitale-signatur)
@@ -519,6 +520,67 @@ Dokumentebene: `WC` (WillClose), `WS` (WillSave), `DS` (DidSave), `WP`
 
 ---
 
+## Ein bestehendes Formular ausfüllen (AcroForm)
+
+Das Obige erstellt ein neues Formular. Um ein von jemand anderem erzeugtes
+PDF zu öffnen — eine hochgeladene Vorlage, ein Behördenformular — und die
+Werte seiner bereits definierten Felder namentlich auszufüllen, verwenden
+Sie stattdessen `ExistingFormFiller`:
+
+```php
+use Dskripchenko\PhpPdf\Pdf\Forms\ExistingFormFiller;
+
+ExistingFormFiller::fromFile('template.pdf')
+    ->setValues([
+        'full_name' => 'Jane Roe',
+        'agree' => 'yes',
+        'employer.name' => 'Acme Corp', // über /Parent-Vererbung aufgelöst
+    ])
+    ->stampImage(0, 'signature.png', x: 100, y: 600, width: 120, height: 40)
+    ->flatten()
+    ->toFile('filled.pdf');
+```
+
+Das Quelldokument wird nie verändert — jeder Aufruf arbeitet auf einer
+tiefen Kopie des *gesamten* Objektgraphen, sodass alles Unberührte
+(Lesezeichen, Metadaten, andere Annotationen) unverändert in die Ausgabe
+übernommen wird. `fields()` liefert für jedes Feld seinen vollständig
+qualifizierten, durch Punkte getrennten Namen, Typ, aktuellen Wert, seinen
+`/TU`-Tooltip (falls vorhanden) und (bei Checkbox/Radio) seine
+Ein-Zustand-Optionsnamen, sodass ein Aufrufer die Form einer Vorlage vor
+dem Ausfüllen ermitteln kann:
+
+```php
+foreach (ExistingFormFiller::fromFile('template.pdf')->fields() as $name => $field) {
+    echo "{$name}: {$field->type} = {$field->value}\n";
+}
+```
+
+Unterstützte Feldtypen: `text`, `text-multiline`, `checkbox`, `radio`. Eine
+Checkbox akzeptiert `on`/`yes`/`true`/`1` (ohne Groß-/Kleinschreibung) oder
+ihren exakten Ein-Zustand-Exportnamen; alles andere hakt sie ab. Der Wert
+einer Radiogruppe muss einem ihrer Options-Exportnamen entsprechen (ohne
+Groß-/Kleinschreibung).
+
+`flatten(?array $fieldNames = null)` bäckt den aktuellen Wert in den
+Seiteninhalt ein und entfernt das interaktive Widget — übergeben Sie eine
+Liste von Namen, um eine Teilmenge zu flatten und den Rest interaktiv zu
+lassen, oder lassen Sie es weg, um alles zu flatten (was auch `/AcroForm`
+entfernt, sobald nichts mehr darin übrig ist). Nur `text`/`text-multiline`-
+Werte werden gezeichnet; andere Feldtypen verlieren nur ihr Widget. Das
+Platzhalter-Erscheinungsbild eines nicht ausgefüllten geflatteten Feldes
+(z. B. ein grauer Hintergrund) wird nicht eingebacken — nur Felder mit
+einem Wert erhalten an ihrer Stelle etwas Gezeichnetes. Lassen Sie Felder
+ungeflatteten (Standard), um das Ergebnis ein gewöhnliches interaktives PDF
+bleiben zu lassen.
+
+`stampImage()`/`stampImageBytes()` platziert ein PNG (mit Alphakanal) oder
+JPEG an einer gegebenen Seite/x/y/Breite/Höhe, unabhängig von jedem Feld —
+nützlich für eine Signatur oder ein Foto, das selbst kein Formularfeld ist.
+Koordinaten sind Punkte mit Ursprung oben links.
+
+---
+
 ## Annotationen
 
 ```php
diff --git a/docs/en/USAGE.md b/docs/en/USAGE.md
index 840ca20..f6ec9ab 100644
--- a/docs/en/USAGE.md
+++ b/docs/en/USAGE.md
@@ -24,6 +24,7 @@ the feature you need.
 - [SVG](#svg)
 - [Hyperlinks and bookmarks](#hyperlinks-and-bookmarks)
 - [Forms (AcroForm)](#forms-acroform)
+- [Fill an existing form (AcroForm)](#fill-an-existing-form-acroform)
 - [Annotations](#annotations)
 - [Encryption](#encryption)
 - [Digital signing](#digital-signing)
@@ -507,6 +508,61 @@ events: `WC` (WillClose), `WS` (WillSave), `DS` (DidSave), `WP`
 
 ---
 
+## Fill an existing form (AcroForm)
+
+The above authors a brand-new form. To open a PDF someone else produced —
+an uploaded template, a government form — and fill in the values of its
+already-defined fields by name, use `ExistingFormFiller` instead:
+
+```php
+use Dskripchenko\PhpPdf\Pdf\Forms\ExistingFormFiller;
+
+ExistingFormFiller::fromFile('template.pdf')
+    ->setValues([
+        'full_name' => 'Jane Roe',
+        'agree' => 'yes',
+        'employer.name' => 'Acme Corp', // resolved via /Parent inheritance
+    ])
+    ->stampImage(0, 'signature.png', x: 100, y: 600, width: 120, height: 40)
+    ->flatten()
+    ->toFile('filled.pdf');
+```
+
+The source document is never mutated — every call works on a deep copy of
+the *entire* object graph, so anything not touched (outlines, metadata,
+other annotations) survives untouched into the output. `fields()` returns
+each field's fully-qualified dotted name, type, current value, its `/TU`
+tooltip (if any) and (for checkbox/radio) its on-state option names, so a
+caller can discover a template's shape before filling it:
+
+```php
+foreach (ExistingFormFiller::fromFile('template.pdf')->fields() as $name => $field) {
+    echo "{$name}: {$field->type} = {$field->value}\n";
+}
+```
+
+Supported field types: `text`, `text-multiline`, `checkbox`, `radio`. A
+checkbox accepts `on`/`yes`/`true`/`1` (case insensitive) or its exact
+on-state export name; anything else unchecks it. A radio group's value must
+match one of its option export names (case insensitive).
+
+`flatten(?array $fieldNames = null)` bakes the current value into the page
+content and removes the interactive widget — pass a list of names to
+flatten a subset and leave the rest interactive, or omit it to flatten
+everything (which also removes `/AcroForm` once nothing is left in it).
+Only `text`/`text-multiline` values are drawn; other field types just lose
+their widget. An unfilled flattened field's placeholder appearance (e.g. a
+grey background) is not baked in — only fields with a value get anything
+drawn in their place. Leave fields unflattened (the default) to keep the
+result an ordinary interactive PDF instead.
+
+`stampImage()`/`stampImageBytes()` place a PNG (with alpha) or JPEG at a
+given page/x/y/width/height, independent of any field — useful for a
+signature or photo that isn't itself a form field. Coordinates are
+top-left-origin points.
+
+---
+
 ## Annotations
 
 ```php
diff --git a/docs/ru/USAGE.md b/docs/ru/USAGE.md
index 2ffa2b1..9ede8d6 100644
--- a/docs/ru/USAGE.md
+++ b/docs/ru/USAGE.md
@@ -24,6 +24,7 @@
 - [SVG](#svg)
 - [Гиперссылки и закладки](#гиперссылки-и-закладки)
 - [Формы (AcroForm)](#формы-acroform)
+- [Заполнение существующей формы (AcroForm)](#заполнение-существующей-формы-acroform)
 - [Аннотации](#аннотации)
 - [Шифрование](#шифрование)
 - [Цифровая подпись](#цифровая-подпись)
@@ -519,6 +520,66 @@ JavaScript-хуки на уровне поля: `keystrokeScript`, `validateScri
 
 ---
 
+## Заполнение существующей формы (AcroForm)
+
+Выше описано создание новой формы. Чтобы открыть PDF, созданный кем-то
+другим — загруженный шаблон, государственную форму — и заполнить значения
+уже определённых в нём полей по имени, используйте вместо этого
+`ExistingFormFiller`:
+
+```php
+use Dskripchenko\PhpPdf\Pdf\Forms\ExistingFormFiller;
+
+ExistingFormFiller::fromFile('template.pdf')
+    ->setValues([
+        'full_name' => 'Jane Roe',
+        'agree' => 'yes',
+        'employer.name' => 'Acme Corp', // разрешается через наследование /Parent
+    ])
+    ->stampImage(0, 'signature.png', x: 100, y: 600, width: 120, height: 40)
+    ->flatten()
+    ->toFile('filled.pdf');
+```
+
+Исходный документ никогда не изменяется — каждый вызов работает с глубокой
+копией *всего* графа объектов, поэтому всё нетронутое (закладки, метаданные,
+другие аннотации) переносится в результат без изменений. `fields()`
+возвращает для каждого поля его полное имя с точками, тип, текущее значение,
+подсказку `/TU` (если есть) и (для чекбокса/радио) имена включённого
+состояния, что позволяет узнать структуру шаблона перед заполнением:
+
+```php
+foreach (ExistingFormFiller::fromFile('template.pdf')->fields() as $name => $field) {
+    echo "{$name}: {$field->type} = {$field->value}\n";
+}
+```
+
+Поддерживаемые типы полей: `text`, `text-multiline`, `checkbox`, `radio`.
+Чекбокс принимает `on`/`yes`/`true`/`1` (без учёта регистра) или точное имя
+экспорта включённого состояния; любое другое значение снимает отметку.
+Значение группы радиокнопок должно совпадать с одним из имён экспорта
+опций (без учёта регистра).
+
+`flatten(?array $fieldNames = null)` впечатывает текущее значение в
+содержимое страницы и удаляет интерактивный виджет — передайте список
+имён, чтобы «сплющить» подмножество и оставить остальные поля
+интерактивными, либо не передавайте ничего, чтобы «сплющить» всё (это
+также удаляет `/AcroForm`, когда в нём ничего не остаётся). Отрисовываются
+только значения `text`/`text-multiline`; остальные типы полей просто
+теряют свой виджет. Внешний вид незаполненного «сплющенного» поля
+(например, серый фон-плейсхолдер) не впечатывается — что-то рисуется на
+его месте только для полей со значением. Оставьте поля не «сплющенными»
+(поведение по умолчанию), чтобы результат остался обычным интерактивным
+PDF.
+
+`stampImage()`/`stampImageBytes()` размещает PNG (с альфа-каналом) или
+JPEG по заданным странице/x/y/ширине/высоте, независимо от каких-либо
+полей — полезно для подписи или фотографии, которая сама по себе не
+является полем формы. Координаты заданы в точках с началом координат
+в левом верхнем углу.
+
+---
+
 ## Аннотации
 
 ```php
diff --git a/docs/zh/USAGE.md b/docs/zh/USAGE.md
index 8c4b4a0..7f59ffa 100644
--- a/docs/zh/USAGE.md
+++ b/docs/zh/USAGE.md
@@ -23,6 +23,7 @@ HTML 转 PDF 一直深入到最底层的页面发射。每个章节都是自包
 - [SVG](#svg)
 - [超链接与书签](#超链接与书签)
 - [表单（AcroForm）](#表单acroform)
+- [填写已有表单（AcroForm）](#填写已有表单acroform)
 - [标注](#标注)
 - [加密](#加密)
 - [数字签名](#数字签名)
@@ -498,6 +499,56 @@ DocumentBuilder::new()
 
 ---
 
+## 填写已有表单（AcroForm）
+
+以上是从零创建新表单。若要打开他人生成的 PDF——上传的模板、政府表格——
+并按名称填写其中已定义字段的值，请改用 `ExistingFormFiller`：
+
+```php
+use Dskripchenko\PhpPdf\Pdf\Forms\ExistingFormFiller;
+
+ExistingFormFiller::fromFile('template.pdf')
+    ->setValues([
+        'full_name' => 'Jane Roe',
+        'agree' => 'yes',
+        'employer.name' => 'Acme Corp', // 通过 /Parent 继承解析
+    ])
+    ->stampImage(0, 'signature.png', x: 100, y: 600, width: 120, height: 40)
+    ->flatten()
+    ->toFile('filled.pdf');
+```
+
+源文档永远不会被修改——每次调用都作用于*整个*对象图的深拷贝，因此未被
+触及的内容（大纲、元数据、其他标注）会原样保留到输出中。`fields()` 会
+返回每个字段的完整点分名称、类型、当前值、`/TU` 提示文字（如果有），以及
+（对复选框/单选按钮而言）其选中状态的导出名称，方便调用方在填写前了解
+模板的结构：
+
+```php
+foreach (ExistingFormFiller::fromFile('template.pdf')->fields() as $name => $field) {
+    echo "{$name}: {$field->type} = {$field->value}\n";
+}
+```
+
+支持的字段类型：`text`、`text-multiline`、`checkbox`、`radio`。复选框接受
+`on`/`yes`/`true`/`1`（不区分大小写）或其精确的选中状态导出名称；其他任何
+值都会取消勾选。单选按钮组的值必须匹配其某个选项的导出名称（不区分大小
+写）。
+
+`flatten(?array $fieldNames = null)` 会将当前值绘制进页面内容并移除交互
+式部件——传入字段名称列表可只压平其中一部分、其余字段保持交互，省略参数
+则压平全部字段（这也会在 `/AcroForm` 中不再留有任何字段时将其整体移除）。
+仅 `text`/`text-multiline` 的值会被绘制；其他字段类型只是失去其部件。未
+填写字段被压平后，其占位外观（例如灰色背景）不会被绘制保留——只有有值的
+字段才会在原位置绘制内容。保持默认（不压平）可使结果仍是一份普通的交互
+式 PDF。
+
+`stampImage()`/`stampImageBytes()` 可在指定的页面/x/y/宽/高处放置一张 PNG
+（保留透明度）或 JPEG 图片，与任何字段无关——适用于签名或照片这类本身并
+非表单字段的内容。坐标以左上角为原点。
+
+---
+
 ## 标注
 
 ```php
diff --git a/scripts/conformance/generate.php b/scripts/conformance/generate.php
index a7fbd65..ec2bf7f 100644
--- a/scripts/conformance/generate.php
+++ b/scripts/conformance/generate.php
@@ -22,6 +22,8 @@
 use Dskripchenko\PhpPdf\Element\Table;
 use Dskripchenko\PhpPdf\Font\Ttf\TtfFile;
 use Dskripchenko\PhpPdf\Layout\Engine;
+use Dskripchenko\PhpPdf\Pdf\Document as PdfDocument;
+use Dskripchenko\PhpPdf\Pdf\Forms\ExistingFormFiller;
 use Dskripchenko\PhpPdf\Pdf\PdfAConfig;
 use Dskripchenko\PhpPdf\Pdf\PdfFont;
 use Dskripchenko\PhpPdf\Pdf\PdfXConfig;
@@ -185,4 +187,28 @@
     $failures += $ok ? 0 : 1;
 }
 
+// Visual fixture for ExistingFormFiller: a small template with a text field
+// and a checkbox, filled, flattened, and stamped with a signature image —
+// exercises the whole fill/flatten/stamp pipeline in one rendered page.
+$template = PdfDocument::new(compressStreams: false);
+$templatePage = $template->addPage();
+$templatePage->addFormField('text', 'full_name', 72, 700, 250, 20, defaultValue: '');
+$templatePage->addFormField('checkbox', 'agree', 72, 660, 14, 14);
+
+$formFillOk = true;
+try {
+    $bytes = ExistingFormFiller::fromBytes($template->toBytes())
+        ->setValue('full_name', 'Jane Roe')
+        ->setValue('agree', 'yes')
+        ->flatten()
+        ->stampImage(0, $root.'/tests/fixtures/1x1.png', 72, 600, 40, 40)
+        ->toBytes();
+    file_put_contents($outDir.'/form-fill.pdf', $bytes);
+    echo "generated $outDir/form-fill.pdf\n";
+} catch (\Throwable $e) {
+    fwrite(STDERR, "FAILED form-fill: {$e->getMessage()}\n");
+    $formFillOk = false;
+}
+$failures += $formFillOk ? 0 : 1;
+
 exit($failures > 0 ? 1 : 0);
diff --git a/scripts/conformance/visual-check.sh b/scripts/conformance/visual-check.sh
index 77e477c..e1ced2d 100755
--- a/scripts/conformance/visual-check.sh
+++ b/scripts/conformance/visual-check.sh
@@ -33,7 +33,7 @@ FUZZ="5%"
 MAX_DIFF_PCT="0.5"
 
 # First page of each document is the visual fixture.
-DOCS=(pdfa-1b pdfx-4)
+DOCS=(pdfa-1b pdfx-4 form-fill)
 
 mkdir -p "$RENDER" "$GOLDEN"
 
diff --git a/scripts/fixtures/generate-parent-inherited-form.php b/scripts/fixtures/generate-parent-inherited-form.php
new file mode 100644
index 0000000..5f8c8e8
--- /dev/null
+++ b/scripts/fixtures/generate-parent-inherited-form.php
@@ -0,0 +1,63 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * Hand-builds a minimal AcroForm PDF with a text field reachable only
+ * through /Parent-chain name inheritance ("employer.name") — a shape the
+ * library's own FormField authoring API cannot produce (it only emits flat
+ * fields, plus its own auto-generated radio-group parent/kids). Used as a
+ * static fixture by tests/Pdf/Forms/ExistingFormFillerTest.php.
+ *
+ * Run: php scripts/fixtures/generate-parent-inherited-form.php
+ */
+
+require __DIR__.'/../../vendor/autoload.php';
+
+use Dskripchenko\PhpPdf\Pdf\Writer;
+
+$writer = new Writer();
+
+$pageId = $writer->reserveObject();
+$pagesId = $writer->reserveObject();
+$catalogId = $writer->reserveObject();
+$acroFormId = $writer->reserveObject();
+$parentId = $writer->reserveObject();
+$childId = $writer->reserveObject();
+
+$contentsId = $writer->addObject("<< /Length 0 >>\nstream\n\nendstream");
+
+// Non-terminal field node: only /T + /Kids, no /FT/value of its own.
+$writer->setObject($parentId, "<< /FT /Tx /T (employer) /Kids [{$childId} 0 R] >>");
+
+// Terminal field, itself the sole widget annotation: partial /T "name" +
+// /Parent -> fully-qualified name resolves to "employer.name".
+$writer->setObject($childId, sprintf(
+    '<< /Type /Annot /Subtype /Widget /FT /Tx /Rect [100 700 300 720] '
+    .'/T (name) /Parent %d 0 R /P %d 0 R /F 4 /V () /DA (/Helv 10 Tf 0 g) >>',
+    $parentId,
+    $pageId,
+));
+
+$writer->setObject($acroFormId, sprintf(
+    '<< /Fields [%d 0 R] /DA (/Helv 10 Tf 0 g) /DR << /Font << /Helv << /Type /Font /Subtype /Type1 /BaseFont /Helvetica >> >> >> >>',
+    $parentId,
+));
+
+$writer->setObject($pageId, sprintf(
+    '<< /Type /Page /Parent %d 0 R /MediaBox [0 0 612 792] /Contents %d 0 R '
+    .'/Resources << /Font << /Helv << /Type /Font /Subtype /Type1 /BaseFont /Helvetica >> >> >> '
+    .'/Annots [%d 0 R] >>',
+    $pagesId,
+    $contentsId,
+    $childId,
+));
+
+$writer->setObject($pagesId, sprintf('<< /Type /Pages /Kids [%d 0 R] /Count 1 >>', $pageId));
+$writer->setObject($catalogId, sprintf('<< /Type /Catalog /Pages %d 0 R /AcroForm %d 0 R >>', $pagesId, $acroFormId));
+$writer->setRoot($catalogId);
+
+$outPath = __DIR__.'/../../tests/fixtures/forms/parent-inherited.pdf';
+file_put_contents($outPath, $writer->toBytes());
+
+echo "Wrote {$outPath}\n";
diff --git a/src/Pdf/Forms/ExistingFormFiller.php b/src/Pdf/Forms/ExistingFormFiller.php
new file mode 100644
index 0000000..cdabc41
--- /dev/null
+++ b/src/Pdf/Forms/ExistingFormFiller.php
@@ -0,0 +1,378 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Dskripchenko\PhpPdf\Pdf\Forms;
+
+use Dskripchenko\PhpPdf\Image\PdfImage;
+use Dskripchenko\PhpPdf\Pdf\Merge\MergeSerializer;
+use Dskripchenko\PhpPdf\Pdf\Merge\ObjectImporter;
+use Dskripchenko\PhpPdf\Pdf\Merge\PdfSource;
+use Dskripchenko\PhpPdf\Pdf\Reader\PdfDictionary;
+use Dskripchenko\PhpPdf\Pdf\Reader\PdfReference;
+use Dskripchenko\PhpPdf\Pdf\Reader\PdfString;
+use Dskripchenko\PhpPdf\Pdf\Reader\ReaderDocument;
+
+/**
+ * Opens an already-existing PDF and fills in the values of its own AcroForm
+ * fields by name — as opposed to {@see \Dskripchenko\PhpPdf\Element\FormField},
+ * which authors a brand-new form from scratch.
+ *
+ * The source document is never mutated: every operation works on a deep copy
+ * (via {@see \Dskripchenko\PhpPdf\Pdf\Merge\ObjectImporter}, the same generic
+ * object-graph copier {@see \Dskripchenko\PhpPdf\Pdf\Merge\PdfMerger} uses for
+ * pages) of the *entire* document graph, so anything not touched — outlines,
+ * metadata, other annotations — survives untouched into the output.
+ *
+ * ```php
+ * $bytes = ExistingFormFiller::fromFile('template.pdf')
+ *     ->setValue('full_name', 'Jane Roe')
+ *     ->setValue('subscribe', 'Yes')
+ *     ->toBytes();
+ * ```
+ */
+final class ExistingFormFiller
+{
+    private readonly PdfSource $source;
+
+    /** @var array<string, FieldNode>|null */
+    private ?array $fieldTree = null;
+
+    private ?ObjectImporter $importer = null;
+
+    private ?int $catalogId = null;
+
+    /** Source object number of `/AcroForm`, or null when the source has none. */
+    private ?int $acroFormObjNum = null;
+
+    private bool $mutated = false;
+
+    private function __construct(PdfSource $source)
+    {
+        $this->source = $source;
+    }
+
+    public static function fromFile(string $path, string $password = ''): self
+    {
+        return new self(PdfSource::fromFile($path, $password));
+    }
+
+    public static function fromBytes(string $bytes, string $password = ''): self
+    {
+        return new self(PdfSource::fromBytes($bytes, $password));
+    }
+
+    /**
+     * Enumerate the source document's AcroForm fields by fully-qualified
+     * name. Always reflects the original source, not pending edits.
+     *
+     * @return array<string, FieldInfo>
+     */
+    public function fields(): array
+    {
+        $out = [];
+        foreach ($this->tree() as $name => $node) {
+            $out[$name] = new FieldInfo(
+                name: $node->name,
+                type: $node->type,
+                value: $node->value,
+                options: $node->options,
+                required: $node->required,
+                readOnly: $node->readOnly,
+                tu: $node->tu,
+            );
+        }
+
+        return $out;
+    }
+
+    /**
+     * Set one field's value by its fully-qualified dotted name (see
+     * {@see fields()}). For a checkbox, any of `on`/`yes`/`true`/`1` (case
+     * insensitive) or the exact on-state export name checks it, anything
+     * else unchecks it. For a radio group, the value must match one of the
+     * group's option export names (case insensitive) to select it.
+     */
+    public function setValue(string $name, string $value): self
+    {
+        $node = $this->tree()[$name] ?? null;
+        if ($node === null) {
+            throw new \InvalidArgumentException("Unknown field: {$name}");
+        }
+
+        $importer = $this->importer();
+        (new FieldValueSetter())->apply($importer, $node, $value);
+        $this->mutated = true;
+
+        return $this;
+    }
+
+    /** @param array<string,string> $values */
+    public function setValues(array $values): self
+    {
+        foreach ($values as $name => $value) {
+            $this->setValue($name, $value);
+        }
+
+        return $this;
+    }
+
+    /**
+     * Bake the current value of the given fields into their page content and
+     * remove their interactive widgets. `null` (default) flattens every
+     * field. When flattening leaves no fields behind, `/AcroForm` is dropped
+     * from the output catalog entirely.
+     *
+     * @param  list<string>|null  $fieldNames
+     */
+    public function flatten(?array $fieldNames = null): self
+    {
+        $tree = $this->tree();
+        $names = $fieldNames ?? array_keys($tree);
+        $importer = $this->importer();
+        $doc = $this->document();
+        $flattener = new FormFlattener();
+        // Both depend only on the fixed /AcroForm object, not on the field
+        // being flattened — compute them once rather than per field.
+        $dr = $this->importedDr($importer);
+        $acroFormDa = $this->acroFormLevelDa($doc);
+
+        foreach ($names as $name) {
+            $node = $tree[$name] ?? null;
+            if ($node === null) {
+                throw new \InvalidArgumentException("Unknown field: {$name}");
+            }
+            $flattener->flatten($importer, $doc, $node, $dr, $acroFormDa);
+            $this->removeFromAcroForm($importer, $node);
+        }
+
+        $this->dropAcroFormIfEmpty($importer);
+
+        return $this;
+    }
+
+    /**
+     * Place a PNG/JPEG image (with alpha preserved for PNG) onto the given
+     * 0-based page, at `$x,$y,$w,$h` in top-left-origin points. Independent
+     * of any AcroForm field — composes with {@see setValue()} and
+     * {@see flatten()} on the same page.
+     */
+    public function stampImage(int $pageIndex, string $imagePath, float $x, float $y, float $width, float $height): self
+    {
+        return $this->stampImageBytes($pageIndex, (string) file_get_contents($imagePath), $x, $y, $width, $height);
+    }
+
+    public function stampImageBytes(int $pageIndex, string $bytes, float $x, float $y, float $width, float $height): self
+    {
+        $doc = $this->document();
+        $pages = $doc->pages();
+        if ($pageIndex < 0 || $pageIndex >= count($pages)) {
+            throw new \OutOfRangeException("Page {$pageIndex} does not exist (document has " . count($pages) . ' pages)');
+        }
+        $sourcePage = $pages[$pageIndex];
+
+        $importer = $this->importer();
+        $pageId = $importer->importObject($sourcePage->objectNumber)->number;
+        $image = PdfImage::fromBytes($bytes);
+
+        (new ImageStamper())->stamp($importer, $pageId, $image, $x, $y, $width, $height, $sourcePage->height());
+
+        return $this;
+    }
+
+    public function toBytes(): string
+    {
+        $importer = $this->importer();
+
+        if ($this->mutated && $this->acroFormObjNum !== null) {
+            $acroFormId = $importer->importObject($this->acroFormObjNum)->number;
+            $acroForm = $importer->get($acroFormId);
+            if ($acroForm instanceof PdfDictionary) {
+                $items = $acroForm->all();
+                $items['NeedAppearances'] = true;
+                $importer->set($acroFormId, new PdfDictionary($items));
+            }
+        }
+
+        return (new MergeSerializer())->serialize($importer->objects(), $this->catalogId());
+    }
+
+    public function toFile(string $path): int
+    {
+        $bytes = $this->toBytes();
+        $written = @file_put_contents($path, $bytes);
+        if ($written === false) {
+            throw new \RuntimeException("Cannot write PDF file: {$path}");
+        }
+
+        return $written;
+    }
+
+    /** @return array<string, FieldNode> */
+    private function tree(): array
+    {
+        return $this->fieldTree ??= (new FieldTree())->build($this->document());
+    }
+
+    private function document(): ReaderDocument
+    {
+        return $this->source->document();
+    }
+
+    private function importer(): ObjectImporter
+    {
+        if ($this->importer !== null) {
+            return $this->importer;
+        }
+
+        $doc = $this->document();
+        $rootRef = $doc->trailer()->get('Root');
+        if (!$rootRef instanceof PdfReference) {
+            throw new \RuntimeException('Document catalog (/Root) is not an indirect reference');
+        }
+
+        $importer = new ObjectImporter();
+        $importer->useSource($doc);
+        $this->catalogId = $importer->importObject($rootRef->number)->number;
+
+        $acroFormRaw = $doc->catalog()->get('AcroForm');
+        $this->acroFormObjNum = $acroFormRaw instanceof PdfReference ? $acroFormRaw->number : null;
+
+        return $this->importer = $importer;
+    }
+
+    private function importedDr(ObjectImporter $importer): ?PdfDictionary
+    {
+        if ($this->acroFormObjNum === null) {
+            return null;
+        }
+        $acroForm = $importer->get($importer->importObject($this->acroFormObjNum)->number);
+        if (!$acroForm instanceof PdfDictionary) {
+            return null;
+        }
+        $dr = $acroForm->get('DR');
+        if ($dr instanceof PdfReference) {
+            $dr = $importer->get($dr->number);
+        }
+
+        return $dr instanceof PdfDictionary ? $dr : null;
+    }
+
+    private function acroFormLevelDa(ReaderDocument $doc): ?string
+    {
+        if ($this->acroFormObjNum === null) {
+            return null;
+        }
+        $acroForm = $doc->deref(new PdfReference($this->acroFormObjNum, 0));
+        if (!$acroForm instanceof PdfDictionary) {
+            return null;
+        }
+        $da = $acroForm->get('DA');
+
+        return $da instanceof PdfString ? $da->bytes : null;
+    }
+
+    private function removeFromAcroForm(ObjectImporter $importer, FieldNode $node): void
+    {
+        $fieldNewId = $importer->importObject($node->fieldObjNum)->number;
+
+        if ($node->parentObjNum !== null) {
+            $parentId = $importer->importObject($node->parentObjNum)->number;
+            $this->removeRefFromArrayKey($importer, $parentId, 'Kids', $fieldNewId);
+
+            // A non-terminal parent left with no /Kids is a dangling field
+            // node — drop it from /AcroForm /Fields too, or dropAcroFormIfEmpty()
+            // would never see the AcroForm as empty once every terminal field
+            // reachable only through it has been flattened.
+            $parentDict = $importer->get($parentId);
+            $kids = $parentDict instanceof PdfDictionary ? $parentDict->get('Kids') : null;
+            if ($kids instanceof PdfReference) {
+                $kids = $importer->get($kids->number);
+            }
+            if (is_array($kids) && $kids === [] && $this->acroFormObjNum !== null) {
+                $acroFormId = $importer->importObject($this->acroFormObjNum)->number;
+                $this->removeRefFromArrayKey($importer, $acroFormId, 'Fields', $parentId);
+            }
+
+            return;
+        }
+
+        if ($this->acroFormObjNum === null) {
+            return;
+        }
+        $acroFormId = $importer->importObject($this->acroFormObjNum)->number;
+        $this->removeRefFromArrayKey($importer, $acroFormId, 'Fields', $fieldNewId);
+    }
+
+    private function removeRefFromArrayKey(ObjectImporter $importer, int $objId, string $key, int $targetNewId): void
+    {
+        $dict = $importer->get($objId);
+        if (!$dict instanceof PdfDictionary) {
+            return;
+        }
+        $arr = $dict->get($key);
+
+        // The array itself may be an indirect object (ISO 32000-1 allows any
+        // value to be indirect) rather than inlined into the dictionary.
+        $arrObjId = null;
+        if ($arr instanceof PdfReference) {
+            $arrObjId = $arr->number;
+            $arr = $importer->get($arrObjId);
+        }
+        if (!is_array($arr)) {
+            return;
+        }
+
+        $filtered = array_values(array_filter(
+            $arr,
+            static fn ($ref) => !($ref instanceof PdfReference && $ref->number === $targetNewId),
+        ));
+
+        if ($arrObjId !== null) {
+            $importer->set($arrObjId, $filtered);
+
+            return;
+        }
+
+        $items = $dict->all();
+        $items[$key] = $filtered;
+        $importer->set($objId, new PdfDictionary($items));
+    }
+
+    private function dropAcroFormIfEmpty(ObjectImporter $importer): void
+    {
+        if ($this->acroFormObjNum === null) {
+            return;
+        }
+        $acroFormId = $importer->importObject($this->acroFormObjNum)->number;
+        $acroForm = $importer->get($acroFormId);
+        if (!$acroForm instanceof PdfDictionary) {
+            return;
+        }
+        $fields = $acroForm->get('Fields');
+        if ($fields instanceof PdfReference) {
+            $fields = $importer->get($fields->number);
+        }
+        if (!is_array($fields) || $fields !== []) {
+            return;
+        }
+
+        $catalog = $importer->get($this->catalogId());
+        if ($catalog instanceof PdfDictionary) {
+            $items = $catalog->all();
+            unset($items['AcroForm']);
+            $importer->set($this->catalogId(), new PdfDictionary($items));
+        }
+        $this->acroFormObjNum = null;
+    }
+
+    private function catalogId(): int
+    {
+        $this->importer(); // ensure catalogId is populated
+        if ($this->catalogId === null) {
+            throw new \LogicException('Catalog was not imported');
+        }
+
+        return $this->catalogId;
+    }
+}
diff --git a/src/Pdf/Forms/FieldInfo.php b/src/Pdf/Forms/FieldInfo.php
new file mode 100644
index 0000000..f429c94
--- /dev/null
+++ b/src/Pdf/Forms/FieldInfo.php
@@ -0,0 +1,31 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Dskripchenko\PhpPdf\Pdf\Forms;
+
+/**
+ * A read-only snapshot of one AcroForm field in a source document, keyed by
+ * its fully-qualified dotted name (ISO 32000-1 §12.7.3.2).
+ */
+final readonly class FieldInfo
+{
+    /**
+     * @param  string  $type  'text', 'text-multiline', 'checkbox', 'radio',
+     *                         or the raw `/FT` name (`Btn`, `Ch`, `Sig`, …)
+     *                         for field types without a more specific mapping.
+     * @param  list<string>  $options  On-state export names for checkbox/radio,
+     *                                  read from the widget(s)' own `/AP/N` keys.
+     */
+    public function __construct(
+        public string $name,
+        public string $type,
+        public string $value,
+        public array $options,
+        public bool $required,
+        public bool $readOnly,
+        /** The field's `/TU` (alternate field name / tooltip), if any. */
+        public ?string $tu = null,
+    ) {
+    }
+}
diff --git a/src/Pdf/Forms/FieldNode.php b/src/Pdf/Forms/FieldNode.php
new file mode 100644
index 0000000..cf7ba8d
--- /dev/null
+++ b/src/Pdf/Forms/FieldNode.php
@@ -0,0 +1,36 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Dskripchenko\PhpPdf\Pdf\Forms;
+
+/**
+ * A resolved terminal AcroForm field found while walking a source document's
+ * `/AcroForm /Fields` tree — carries the source object numbers needed to
+ * later locate and mutate the field once it has been copied by an
+ * {@see \Dskripchenko\PhpPdf\Pdf\Merge\ObjectImporter}.
+ */
+final readonly class FieldNode
+{
+    /**
+     * @param  list<int>  $widgetObjNums  Source object numbers of the field's
+     *                                     own widget annotation(s) — a single
+     *                                     entry for a plain field, or one per
+     *                                     option for a radio group.
+     */
+    public function __construct(
+        public string $name,
+        public string $type,
+        public int $fieldObjNum,
+        public array $widgetObjNums,
+        public ?int $parentObjNum,
+        public ?string $da,
+        public ?string $tu,
+        public string $value,
+        /** @var list<string> */
+        public array $options,
+        public bool $required,
+        public bool $readOnly,
+    ) {
+    }
+}
diff --git a/src/Pdf/Forms/FieldTree.php b/src/Pdf/Forms/FieldTree.php
new file mode 100644
index 0000000..8435f79
--- /dev/null
+++ b/src/Pdf/Forms/FieldTree.php
@@ -0,0 +1,222 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Dskripchenko\PhpPdf\Pdf\Forms;
+
+use Dskripchenko\PhpPdf\Pdf\Reader\PdfDictionary;
+use Dskripchenko\PhpPdf\Pdf\Reader\PdfName;
+use Dskripchenko\PhpPdf\Pdf\Reader\PdfReference;
+use Dskripchenko\PhpPdf\Pdf\Reader\PdfString;
+use Dskripchenko\PhpPdf\Pdf\Reader\ReaderDocument;
+
+/**
+ * Walks a source document's `/AcroForm /Fields` tree, resolving each
+ * terminal field's fully-qualified dotted name from its `/Parent` chain
+ * (ISO 32000-1 §12.7.3.2) and inheriting `/FT`, `/DA` and `/Ff` from
+ * ancestors that don't redeclare them.
+ *
+ * A `/Kids` entry is a *child field* (recursed into) when it has its own
+ * `/T`; a `/Kids` entry without `/T` is just one more widget annotation of
+ * the current field (e.g. one option of a radio group, or the same field
+ * repeated on several pages) rather than a new name level.
+ */
+final class FieldTree
+{
+    private const MAX_DEPTH = 64;
+
+    /** @return array<string, FieldNode> keyed by fully-qualified name */
+    public function build(ReaderDocument $doc): array
+    {
+        $acroForm = $doc->deref($doc->catalog()->get('AcroForm'));
+        if (!$acroForm instanceof PdfDictionary) {
+            return [];
+        }
+
+        $fields = $doc->deref($acroForm->get('Fields'));
+        if (!is_array($fields)) {
+            return [];
+        }
+
+        $out = [];
+        foreach ($fields as $ref) {
+            $this->walk($doc, $ref, null, null, null, null, null, $out, 0);
+        }
+
+        return $out;
+    }
+
+    /**
+     * @param array<string, FieldNode> $out
+     */
+    private function walk(
+        ReaderDocument $doc,
+        mixed $ref,
+        ?string $parentName,
+        ?int $parentObjNum,
+        ?string $inheritedFt,
+        ?string $inheritedDa,
+        ?int $inheritedFf,
+        array &$out,
+        int $depth,
+    ): void {
+        if ($depth > self::MAX_DEPTH || !$ref instanceof PdfReference) {
+            return;
+        }
+        $dict = $doc->deref($ref);
+        if (!$dict instanceof PdfDictionary) {
+            return;
+        }
+
+        $partial = $this->decodeTextString($dict->get('T'));
+        $name = $partial !== null
+            ? ($parentName !== null ? $parentName . '.' . $partial : $partial)
+            : $parentName;
+
+        $ft = $this->nameValue($dict->get('FT')) ?? $inheritedFt;
+        $da = $dict->get('DA') instanceof PdfString ? $this->decodeName($dict->get('DA')) : $inheritedDa;
+        $ff = is_int($dict->get('Ff')) ? $dict->get('Ff') : $inheritedFf;
+        $tu = $this->decodeTextString($dict->get('TU'));
+
+        $kids = $doc->deref($dict->get('Kids'));
+        $childFieldRefs = [];
+        $widgetRefs = [];
+        if (is_array($kids)) {
+            foreach ($kids as $kidRef) {
+                $kidDict = $doc->deref($kidRef);
+                if ($kidDict instanceof PdfDictionary && $kidDict->has('T')) {
+                    $childFieldRefs[] = $kidRef;
+                } else {
+                    $widgetRefs[] = $kidRef;
+                }
+            }
+        }
+
+        if ($childFieldRefs !== []) {
+            foreach ($childFieldRefs as $kidRef) {
+                $this->walk($doc, $kidRef, $name, $ref->number, $ft, $da, $ff, $out, $depth + 1);
+            }
+            return;
+        }
+
+        if ($name === null || $ft === null) {
+            return;
+        }
+
+        $widgetObjNums = [];
+        if ($widgetRefs !== []) {
+            foreach ($widgetRefs as $widgetRef) {
+                if ($widgetRef instanceof PdfReference) {
+                    $widgetObjNums[] = $widgetRef->number;
+                }
+            }
+        } else {
+            $widgetObjNums = [$ref->number];
+        }
+
+        $type = $this->resolveType($ft, $ff ?? 0);
+        $value = $this->stringValue($doc->deref($dict->get('V')));
+        $options = $this->collectOptions($doc, $widgetObjNums);
+
+        $out[$name] = new FieldNode(
+            name: $name,
+            type: $type,
+            fieldObjNum: $ref->number,
+            widgetObjNums: $widgetObjNums,
+            parentObjNum: $parentObjNum,
+            da: $da,
+            tu: $tu,
+            value: $value,
+            options: $options,
+            required: (($ff ?? 0) & 2) !== 0,
+            readOnly: (($ff ?? 0) & 1) !== 0,
+        );
+    }
+
+    private function resolveType(string $ft, int $ff): string
+    {
+        return match ($ft) {
+            'Tx' => ($ff & 4096) !== 0 ? 'text-multiline' : 'text',
+            'Btn' => match (true) {
+                ($ff & 32768) !== 0 => 'radio',
+                ($ff & 65536) !== 0 => 'push',
+                default => 'checkbox',
+            },
+            'Ch' => ($ff & 131072) !== 0 ? 'combo' : 'list',
+            default => $ft,
+        };
+    }
+
+    /**
+     * @param  list<int>  $widgetObjNums
+     * @return list<string>
+     */
+    private function collectOptions(ReaderDocument $doc, array $widgetObjNums): array
+    {
+        $options = [];
+        foreach ($widgetObjNums as $objNum) {
+            $widget = $doc->deref(new PdfReference($objNum, 0));
+            if (!$widget instanceof PdfDictionary) {
+                continue;
+            }
+            $ap = $doc->deref($widget->get('AP'));
+            if (!$ap instanceof PdfDictionary) {
+                continue;
+            }
+            $n = $doc->deref($ap->get('N'));
+            if (!$n instanceof PdfDictionary) {
+                continue;
+            }
+            foreach (array_keys($n->all()) as $key) {
+                if ($key !== 'Off' && !in_array($key, $options, true)) {
+                    $options[] = $key;
+                }
+            }
+        }
+
+        return $options;
+    }
+
+    private function nameValue(mixed $value): ?string
+    {
+        return $value instanceof PdfName ? $value->value : null;
+    }
+
+    private function decodeName(mixed $value): ?string
+    {
+        return $value instanceof PdfString ? $value->bytes : null;
+    }
+
+    /**
+     * Decodes a PDF text string (ISO 32000-1 §7.9.2.2): UTF-16BE with a
+     * `\xFE\xFF` byte order mark, or PDFDocEncoding (ASCII-compatible for
+     * the characters this library writes) when the BOM is absent.
+     */
+    private function decodeTextString(mixed $value): ?string
+    {
+        if (!$value instanceof PdfString) {
+            return null;
+        }
+
+        $bytes = $value->bytes;
+        if (!str_starts_with($bytes, "\xFE\xFF")) {
+            return $bytes;
+        }
+
+        $decoded = @iconv('UTF-16BE', 'UTF-8', substr($bytes, 2));
+
+        return $decoded !== false ? $decoded : $bytes;
+    }
+
+    private function stringValue(mixed $value): string
+    {
+        if ($value instanceof PdfString) {
+            return $value->bytes;
+        }
+        if ($value instanceof PdfName) {
+            return $value->value;
+        }
+
+        return '';
+    }
+}
diff --git a/src/Pdf/Forms/FieldValueSetter.php b/src/Pdf/Forms/FieldValueSetter.php
new file mode 100644
index 0000000..d19dd4d
--- /dev/null
+++ b/src/Pdf/Forms/FieldValueSetter.php
@@ -0,0 +1,156 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Dskripchenko\PhpPdf\Pdf\Forms;
+
+use Dskripchenko\PhpPdf\Pdf\Merge\ObjectImporter;
+use Dskripchenko\PhpPdf\Pdf\Reader\PdfDictionary;
+use Dskripchenko\PhpPdf\Pdf\Reader\PdfName;
+use Dskripchenko\PhpPdf\Pdf\Reader\PdfString;
+
+/**
+ * Applies a value to one already-imported (deep-copied) AcroForm field,
+ * mutating the copies held by an {@see ObjectImporter} in place. The source
+ * document is never touched — `$importer->importObject()` deduplicates, so
+ * re-requesting a field or widget already copied returns the same new id.
+ */
+final class FieldValueSetter
+{
+    /**
+     * Values considered "checked" for a checkbox when no exact export-name
+     * match is found among the widget's own `/AP/N` keys.
+     */
+    private const TRUTHY = ['on', 'yes', 'true', '1'];
+
+    public function apply(ObjectImporter $importer, FieldNode $node, string $value): void
+    {
+        match ($node->type) {
+            'checkbox' => $this->applyCheckbox($importer, $node, $value),
+            'radio' => $this->applyRadio($importer, $node, $value),
+            default => $this->applyText($importer, $node, $value),
+        };
+    }
+
+    private function applyText(ObjectImporter $importer, FieldNode $node, string $value): void
+    {
+        $fieldId = $importer->importObject($node->fieldObjNum)->number;
+        $this->setItems($importer, $fieldId, ['V' => new PdfString($value)]);
+        // Drop any stale appearance so /NeedAppearances forces the reader to
+        // regenerate it from the new /V rather than showing the old value.
+        $this->removeAp($importer, $fieldId);
+
+        foreach ($node->widgetObjNums as $widgetObjNum) {
+            if ($widgetObjNum === $node->fieldObjNum) {
+                continue;
+            }
+            $this->removeAp($importer, $importer->importObject($widgetObjNum)->number);
+        }
+    }
+
+    private function applyCheckbox(ObjectImporter $importer, FieldNode $node, string $value): void
+    {
+        $state = 'Off';
+        foreach ($node->widgetObjNums as $widgetObjNum) {
+            $widgetId = $importer->importObject($widgetObjNum)->number;
+            $onKey = $this->onStateKey($importer, $widgetId);
+            if ($onKey !== null && $this->matches($value, $onKey)) {
+                $state = $onKey;
+            }
+        }
+
+        // A second pass, since every widget must agree on the same state
+        // (e.g. the same checkbox repeated on several pages) even though
+        // only one of them may own the matching /AP/N export name.
+        foreach ($node->widgetObjNums as $widgetObjNum) {
+            $widgetId = $importer->importObject($widgetObjNum)->number;
+            $onKey = $this->onStateKey($importer, $widgetId);
+            $this->setItems($importer, $widgetId, ['AS' => new PdfName($onKey === $state ? $state : 'Off')]);
+        }
+
+        $fieldId = $importer->importObject($node->fieldObjNum)->number;
+        $this->setItems($importer, $fieldId, ['V' => new PdfName($state)]);
+    }
+
+    private function applyRadio(ObjectImporter $importer, FieldNode $node, string $value): void
+    {
+        $selected = null;
+        foreach ($node->widgetObjNums as $widgetObjNum) {
+            $widgetId = $importer->importObject($widgetObjNum)->number;
+            $onKey = $this->onStateKey($importer, $widgetId);
+            // Exact match only: unlike a checkbox, a radio group has more
+            // than one "on" state, so the generic yes/true/1 TRUTHY aliases
+            // (meant for a single on/off toggle) must not spuriously match
+            // whichever option happens to be checked last in widget order.
+            if ($onKey !== null && strcasecmp($value, $onKey) === 0) {
+                $selected = $onKey;
+            }
+        }
+
+        foreach ($node->widgetObjNums as $widgetObjNum) {
+            $widgetId = $importer->importObject($widgetObjNum)->number;
+            $onKey = $this->onStateKey($importer, $widgetId);
+            $state = ($selected !== null && $onKey === $selected) ? $selected : 'Off';
+            $this->setItems($importer, $widgetId, ['AS' => new PdfName($state)]);
+        }
+
+        $fieldId = $importer->importObject($node->fieldObjNum)->number;
+        $this->setItems($importer, $fieldId, ['V' => new PdfName($selected ?? 'Off')]);
+    }
+
+    private function matches(string $value, string $onKey): bool
+    {
+        if (strcasecmp($value, $onKey) === 0) {
+            return true;
+        }
+
+        return in_array(strtolower($value), self::TRUTHY, true);
+    }
+
+    /**
+     * The single non-`Off` key of a button widget's own `/AP/N` sub-dictionary
+     * — the export name a reader shows when that widget is selected. Not read
+     * from a fixed convention, since it's author-defined per widget.
+     */
+    private function onStateKey(ObjectImporter $importer, int $widgetId): ?string
+    {
+        $widget = $importer->get($widgetId);
+        if (!$widget instanceof PdfDictionary) {
+            return null;
+        }
+        $ap = $widget->get('AP');
+        if (!$ap instanceof PdfDictionary) {
+            return null;
+        }
+        $n = $ap->get('N');
+        if (!$n instanceof PdfDictionary) {
+            return null;
+        }
+        foreach (array_keys($n->all()) as $key) {
+            if ($key !== 'Off') {
+                return $key;
+            }
+        }
+
+        return null;
+    }
+
+    private function removeAp(ObjectImporter $importer, int $objId): void
+    {
+        $dict = $importer->get($objId);
+        if (!$dict instanceof PdfDictionary || !$dict->has('AP')) {
+            return;
+        }
+        $items = $dict->all();
+        unset($items['AP']);
+        $importer->set($objId, new PdfDictionary($items));
+    }
+
+    /** @param array<string,mixed> $changes */
+    private function setItems(ObjectImporter $importer, int $objId, array $changes): void
+    {
+        $dict = $importer->get($objId);
+        $items = $dict instanceof PdfDictionary ? $dict->all() : [];
+        $importer->set($objId, new PdfDictionary(array_merge($items, $changes)));
+    }
+}
diff --git a/src/Pdf/Forms/FormFlattener.php b/src/Pdf/Forms/FormFlattener.php
new file mode 100644
index 0000000..cc32a3d
--- /dev/null
+++ b/src/Pdf/Forms/FormFlattener.php
@@ -0,0 +1,299 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Dskripchenko\PhpPdf\Pdf\Forms;
+
+use Dskripchenko\PhpPdf\Pdf\Merge\ObjectImporter;
+use Dskripchenko\PhpPdf\Pdf\Reader\PdfDictionary;
+use Dskripchenko\PhpPdf\Pdf\Reader\PdfName;
+use Dskripchenko\PhpPdf\Pdf\Reader\PdfReference;
+use Dskripchenko\PhpPdf\Pdf\Reader\PdfStream;
+use Dskripchenko\PhpPdf\Pdf\Reader\PdfString;
+use Dskripchenko\PhpPdf\Pdf\Reader\ReaderDocument;
+
+/**
+ * Bakes one field's current value into its page's content stream and
+ * removes the interactive widget annotation, on an already-imported (deep
+ * copied) object graph.
+ *
+ * Only text/text-multiline fields get their value drawn — the appearance is
+ * built from the field's own `/DA` (font/size, resolved via the AcroForm's
+ * imported `/DR` so no font is redeclared) and its widget's `/Rect`, single
+ * line or naive fixed-leading multi-line. Other field types (checkbox,
+ * radio, choice, …) just lose their widget annotation; baking in a checked
+ * glyph or a choice's display text is not attempted here.
+ *
+ * Known limitation carried over from the original spike: an *unfilled*
+ * flattened field only has its annotation removed — its placeholder
+ * appearance (e.g. an `/MK /BG` background) is not baked in, so it
+ * disappears rather than surviving as static content the way pdftk's
+ * flatten does.
+ */
+final class FormFlattener
+{
+    public function flatten(
+        ObjectImporter $importer,
+        ReaderDocument $doc,
+        FieldNode $node,
+        ?PdfDictionary $importedDr,
+        ?string $acroFormDa,
+    ): void {
+        // Read the field's *current* value from the already-imported copy,
+        // not the FieldNode snapshot — a prior setValue() call has already
+        // mutated the copy, and the snapshot reflects only the source.
+        $fieldDict = $importer->get($importer->importObject($node->fieldObjNum)->number);
+        $currentValue = $fieldDict instanceof PdfDictionary ? $this->stringValue($fieldDict->get('V')) : $node->value;
+
+        foreach ($node->widgetObjNums as $widgetObjNum) {
+            $this->flattenWidget($importer, $doc, $node, $currentValue, $widgetObjNum, $importedDr, $acroFormDa);
+        }
+    }
+
+    private function flattenWidget(
+        ObjectImporter $importer,
+        ReaderDocument $doc,
+        FieldNode $node,
+        string $currentValue,
+        int $widgetObjNum,
+        ?PdfDictionary $importedDr,
+        ?string $acroFormDa,
+    ): void {
+        $sourcePageObjNum = $this->findSourcePage($doc, $widgetObjNum);
+        if ($sourcePageObjNum === null) {
+            return;
+        }
+        $pageId = $importer->importObject($sourcePageObjNum)->number;
+        $widgetId = $importer->importObject($widgetObjNum)->number;
+        $widget = $importer->get($widgetId);
+        if (!$widget instanceof PdfDictionary) {
+            return;
+        }
+
+        if (in_array($node->type, ['text', 'text-multiline'], true) && $currentValue !== '') {
+            $rect = $this->rect($widget);
+            if ($rect !== null) {
+                // buildContentStream() always returns a stream (it draws
+                // whatever font it could resolve, or none at all), so there
+                // is no null case to guard against here.
+                [$content, $fontKey, $fontRef] = $this->buildContentStream($node, $currentValue, $rect, $node->da ?? $acroFormDa, $importedDr);
+                $this->appendContent($importer, $pageId, $content, $fontKey, $fontRef);
+            }
+        }
+
+        $this->removeAnnotation($importer, $pageId, $widgetId);
+    }
+
+    private function stringValue(mixed $value): string
+    {
+        if ($value instanceof PdfString) {
+            return $value->bytes;
+        }
+        if ($value instanceof PdfName) {
+            return $value->value;
+        }
+
+        return '';
+    }
+
+    private function findSourcePage(ReaderDocument $doc, int $widgetObjNum): ?int
+    {
+        $widget = $doc->deref(new PdfReference($widgetObjNum, 0));
+        if ($widget instanceof PdfDictionary) {
+            $p = $widget->get('P');
+            if ($p instanceof PdfReference) {
+                return $p->number;
+            }
+        }
+        foreach ($doc->pages() as $page) {
+            $annots = $doc->deref($page->dict->get('Annots'));
+            if (!is_array($annots)) {
+                continue;
+            }
+            foreach ($annots as $ref) {
+                if ($ref instanceof PdfReference && $ref->number === $widgetObjNum) {
+                    return $page->objectNumber;
+                }
+            }
+        }
+
+        return null;
+    }
+
+    /** @return array{float,float,float,float}|null */
+    private function rect(PdfDictionary $widget): ?array
+    {
+        $rect = $widget->get('Rect');
+        if (!is_array($rect) || count($rect) !== 4) {
+            return null;
+        }
+        $values = array_map(static fn ($v) => is_numeric($v) ? (float) $v : null, $rect);
+        if (in_array(null, $values, true)) {
+            return null;
+        }
+
+        return [$values[0], $values[1], $values[2], $values[3]];
+    }
+
+    /**
+     * @param  array{float,float,float,float}  $rect
+     * @return array{string, ?string, ?PdfReference}
+     */
+    private function buildContentStream(FieldNode $node, string $value, array $rect, ?string $da, ?PdfDictionary $importedDr): array
+    {
+        [$fontKey, $fontSize] = $this->parseDa($da);
+        $fontRef = null;
+        if ($fontKey !== null && $importedDr instanceof PdfDictionary) {
+            $fonts = $importedDr->get('Font');
+            if ($fonts instanceof PdfDictionary) {
+                $ref = $fonts->get($fontKey);
+                if ($ref instanceof PdfReference) {
+                    $fontRef = $ref;
+                }
+            }
+        }
+
+        [$llx, $lly, , $ury] = $rect;
+        $lines = $node->type === 'text-multiline' ? explode("\n", $value) : [$value];
+        $leading = $fontSize * 1.15;
+
+        $ops = ['q', 'BT'];
+        if ($fontKey !== null && $fontRef !== null) {
+            $ops[] = sprintf('/%s %s Tf', $fontKey, $this->fmt($fontSize));
+        }
+        $ops[] = '0 g';
+        $x = $llx + 2.0;
+        $y = $node->type === 'text-multiline'
+            ? $ury - $fontSize - 2.0
+            : $lly + max(0.0, ($ury - $lly - $fontSize) / 2) + 1.0;
+        $ops[] = sprintf('%s %s Td', $this->fmt($x), $this->fmt($y));
+        foreach ($lines as $i => $line) {
+            if ($i > 0) {
+                $ops[] = sprintf('0 %s Td', $this->fmt(-$leading));
+            }
+            $ops[] = sprintf('(%s) Tj', $this->escape($line));
+        }
+        $ops[] = 'ET';
+        $ops[] = 'Q';
+
+        return [implode("\n", $ops), $fontKey, $fontRef];
+    }
+
+    /** @return array{?string, float} */
+    private function parseDa(?string $da): array
+    {
+        if ($da !== null && preg_match('@/(\S+)\s+([\d.eE+-]+)\s+Tf@', $da, $m) === 1) {
+            return [$m[1], (float) $m[2]];
+        }
+
+        return [null, 9.0];
+    }
+
+    private function escape(string $text): string
+    {
+        $out = '';
+        for ($i = 0; $i < strlen($text); $i++) {
+            $c = $text[$i];
+            $ord = ord($c);
+            if ($c === '\\' || $c === '(' || $c === ')') {
+                $out .= '\\'.$c;
+            } elseif ($ord < 0x20 || $ord > 0x7E) {
+                $out .= sprintf('\\%03o', $ord);
+            } else {
+                $out .= $c;
+            }
+        }
+
+        return $out;
+    }
+
+    private function fmt(float $v): string
+    {
+        if ($v === floor($v) && abs($v) < 1e9) {
+            return (string) (int) $v;
+        }
+
+        return rtrim(rtrim(sprintf('%.4f', $v), '0'), '.');
+    }
+
+    private function appendContent(ObjectImporter $importer, int $pageId, string $content, ?string $fontKey, ?PdfReference $fontRef): void
+    {
+        $dict = $importer->get($pageId);
+        if (!$dict instanceof PdfDictionary) {
+            return;
+        }
+        $items = $dict->all();
+
+        $contents = $items['Contents'] ?? null;
+        $list = match (true) {
+            is_array($contents) => array_values($contents),
+            $contents !== null => [$contents],
+            default => [],
+        };
+        $list[] = new PdfReference($importer->allocate(new PdfStream(new PdfDictionary([]), $content)), 0);
+        $items['Contents'] = $list;
+
+        if ($fontKey !== null && $fontRef !== null) {
+            $resources = $this->resolveDict($importer, $items['Resources'] ?? null) ?? new PdfDictionary([]);
+            $fonts = $this->resolveDict($importer, $resources->get('Font')) ?? new PdfDictionary([]);
+            $fontItems = $fonts->all();
+            if (!isset($fontItems[$fontKey])) {
+                $fontItems[$fontKey] = $fontRef;
+                $resItems = $resources->all();
+                $resItems['Font'] = new PdfDictionary($fontItems);
+                $resources = new PdfDictionary($resItems);
+            }
+            $items['Resources'] = $resources;
+        }
+
+        $importer->set($pageId, new PdfDictionary($items));
+    }
+
+    private function removeAnnotation(ObjectImporter $importer, int $pageId, int $widgetId): void
+    {
+        $dict = $importer->get($pageId);
+        if (!$dict instanceof PdfDictionary) {
+            return;
+        }
+        $annots = $this->resolveArray($importer, $dict->get('Annots'));
+        if ($annots === null) {
+            return;
+        }
+        $filtered = array_values(array_filter(
+            $annots,
+            static fn ($ref) => !($ref instanceof PdfReference && $ref->number === $widgetId),
+        ));
+        $items = $dict->all();
+        $items['Annots'] = $filtered;
+        $importer->set($pageId, new PdfDictionary($items));
+    }
+
+    /**
+     * A dictionary value already read off an imported object may itself be an
+     * indirect reference (ISO 32000-1 allows any value to be indirect) rather
+     * than inlined — resolve it against the importer's own object map before
+     * treating an absent match as "no such dictionary".
+     */
+    private function resolveDict(ObjectImporter $importer, mixed $value): ?PdfDictionary
+    {
+        if ($value instanceof PdfReference) {
+            $value = $importer->get($value->number);
+        }
+
+        return $value instanceof PdfDictionary ? $value : null;
+    }
+
+    /**
+     * Same as {@see resolveDict()}, but for an array-valued entry (e.g. /Annots).
+     *
+     * @return list<mixed>|null
+     */
+    private function resolveArray(ObjectImporter $importer, mixed $value): ?array
+    {
+        if ($value instanceof PdfReference) {
+            $value = $importer->get($value->number);
+        }
+
+        return is_array($value) ? $value : null;
+    }
+}
diff --git a/src/Pdf/Forms/ImageStamper.php b/src/Pdf/Forms/ImageStamper.php
new file mode 100644
index 0000000..fe51883
--- /dev/null
+++ b/src/Pdf/Forms/ImageStamper.php
@@ -0,0 +1,134 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Dskripchenko\PhpPdf\Pdf\Forms;
+
+use Dskripchenko\PhpPdf\Image\PdfImage;
+use Dskripchenko\PhpPdf\Pdf\Merge\ObjectImporter;
+use Dskripchenko\PhpPdf\Pdf\Reader\PdfDictionary;
+use Dskripchenko\PhpPdf\Pdf\Reader\PdfName;
+use Dskripchenko\PhpPdf\Pdf\Reader\PdfReference;
+use Dskripchenko\PhpPdf\Pdf\Reader\PdfStream;
+
+/**
+ * Places a raster image (PNG/JPEG, with alpha preserved via `/SMask`) onto a
+ * page of an already-imported object graph, as its own `/XObject` — a
+ * standalone primitive independent of any AcroForm field, so a caller can
+ * drive it with whatever placement convention it likes (a signature, a
+ * photo, a watermark, …).
+ *
+ * Decoding reuses {@see PdfImage} (already used by the authoring side); only
+ * XObject registration differs, since here objects are `allocate()`d on a
+ * shared {@see ObjectImporter} instead of written via the authoring `Writer`
+ * — mirrors the pattern in {@see \Dskripchenko\PhpPdf\Pdf\Merge\PageXObjectBuilder}.
+ *
+ * Input coordinates are top-left origin (matching how callers already
+ * measure a page for placement), converted here to PDF's bottom-left origin.
+ */
+final class ImageStamper
+{
+    public function stamp(
+        ObjectImporter $importer,
+        int $pageId,
+        PdfImage $image,
+        float $x,
+        float $y,
+        float $width,
+        float $height,
+        float $pageHeight,
+    ): void {
+        $imageId = $this->registerImage($importer, $image);
+        $name = 'Stamp' . $imageId;
+
+        $pdfY = $pageHeight - $y - $height;
+        $cm = sprintf(
+            '%s 0 0 %s %s %s cm',
+            $this->fmt($width), $this->fmt($height), $this->fmt($x), $this->fmt($pdfY),
+        );
+        $content = "q\n{$cm}\n/{$name} Do\nQ";
+
+        $this->appendToPage($importer, $pageId, $content, $name, new PdfReference($imageId, 0));
+    }
+
+    private function registerImage(ObjectImporter $importer, PdfImage $image): int
+    {
+        $items = [
+            'Type' => new PdfName('XObject'),
+            'Subtype' => new PdfName('Image'),
+            'Width' => $image->widthPx,
+            'Height' => $image->heightPx,
+            'ColorSpace' => new PdfName(ltrim($image->colorSpace, '/')),
+            'BitsPerComponent' => $image->bitsPerComponent,
+            'Filter' => new PdfName(ltrim($image->filter, '/')),
+        ];
+
+        if ($image->alphaData !== null && $image->alphaData !== '') {
+            $maskDict = new PdfDictionary([
+                'Type' => new PdfName('XObject'),
+                'Subtype' => new PdfName('Image'),
+                'Width' => $image->widthPx,
+                'Height' => $image->heightPx,
+                'ColorSpace' => new PdfName('DeviceGray'),
+                'BitsPerComponent' => 8,
+                'Filter' => new PdfName('FlateDecode'),
+            ]);
+            $maskId = $importer->allocate(new PdfStream($maskDict, $image->alphaData));
+            $items['SMask'] = new PdfReference($maskId, 0);
+        }
+
+        return $importer->allocate(new PdfStream(new PdfDictionary($items), $image->imageData));
+    }
+
+    private function appendToPage(ObjectImporter $importer, int $pageId, string $content, string $name, PdfReference $imageRef): void
+    {
+        $dict = $importer->get($pageId);
+        if (!$dict instanceof PdfDictionary) {
+            throw new \InvalidArgumentException("Page object {$pageId} was not imported");
+        }
+        $items = $dict->all();
+
+        $contents = $items['Contents'] ?? null;
+        $list = match (true) {
+            is_array($contents) => array_values($contents),
+            $contents !== null => [$contents],
+            default => [],
+        };
+        $list[] = new PdfReference($importer->allocate(new PdfStream(new PdfDictionary([]), $content)), 0);
+        $items['Contents'] = $list;
+
+        $resources = $this->resolveDict($importer, $items['Resources'] ?? null) ?? new PdfDictionary([]);
+        $xobjects = $this->resolveDict($importer, $resources->get('XObject')) ?? new PdfDictionary([]);
+        $xobjItems = $xobjects->all();
+        $xobjItems[$name] = $imageRef;
+        $resItems = $resources->all();
+        $resItems['XObject'] = new PdfDictionary($xobjItems);
+        $items['Resources'] = new PdfDictionary($resItems);
+
+        $importer->set($pageId, new PdfDictionary($items));
+    }
+
+    /**
+     * A dictionary value already read off an imported object may itself be an
+     * indirect reference (ISO 32000-1 allows any value to be indirect) rather
+     * than inlined — resolve it against the importer's own object map before
+     * treating an absent match as "no such dictionary".
+     */
+    private function resolveDict(ObjectImporter $importer, mixed $value): ?PdfDictionary
+    {
+        if ($value instanceof PdfReference) {
+            $value = $importer->get($value->number);
+        }
+
+        return $value instanceof PdfDictionary ? $value : null;
+    }
+
+    private function fmt(float $v): string
+    {
+        if ($v === floor($v) && abs($v) < 1e9) {
+            return (string) (int) $v;
+        }
+
+        return rtrim(rtrim(sprintf('%.4f', $v), '0'), '.');
+    }
+}
diff --git a/tests/Pdf/Forms/ExistingFormFillerTest.php b/tests/Pdf/Forms/ExistingFormFillerTest.php
new file mode 100644
index 0000000..87093e9
--- /dev/null
+++ b/tests/Pdf/Forms/ExistingFormFillerTest.php
@@ -0,0 +1,372 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Dskripchenko\PhpPdf\Tests\Pdf\Forms;
+
+use Dskripchenko\PhpPdf\Pdf\Document as PdfDocument;
+use Dskripchenko\PhpPdf\Pdf\Forms\ExistingFormFiller;
+use Dskripchenko\PhpPdf\Pdf\Forms\FieldTree;
+use Dskripchenko\PhpPdf\Pdf\Reader\PdfName;
+use Dskripchenko\PhpPdf\Pdf\Reader\PdfReference;
+use Dskripchenko\PhpPdf\Pdf\Reader\PdfStream;
+use Dskripchenko\PhpPdf\Pdf\Reader\ReaderDocument;
+use PHPUnit\Framework\Attributes\Test;
+use PHPUnit\Framework\TestCase;
+
+/**
+ * Fill an existing PDF's own AcroForm (a template designed elsewhere), as
+ * opposed to {@see \Dskripchenko\PhpPdf\Element\FormField}, which authors a
+ * new form from scratch.
+ */
+final class ExistingFormFillerTest extends TestCase
+{
+    private function textFieldPdf(bool $objStm = false): string
+    {
+        $pdf = PdfDocument::new(compressStreams: false);
+        if ($objStm) {
+            $pdf->useObjectStreams(true);
+        }
+        $page = $pdf->addPage();
+        $page->addFormField('text', 'full_name', 100, 700, 200, 20, defaultValue: 'Jane Roe');
+
+        return $pdf->toBytes();
+    }
+
+    #[Test]
+    public function enumerates_a_text_field(): void
+    {
+        $filler = ExistingFormFiller::fromBytes($this->textFieldPdf());
+
+        $fields = $filler->fields();
+
+        self::assertArrayHasKey('full_name', $fields);
+        $field = $fields['full_name'];
+        self::assertSame('full_name', $field->name);
+        self::assertSame('text', $field->type);
+        self::assertSame('Jane Roe', $field->value);
+        self::assertFalse($field->required);
+        self::assertFalse($field->readOnly);
+    }
+
+    #[Test]
+    public function enumerates_fields_from_a_compressed_xref_source(): void
+    {
+        $filler = ExistingFormFiller::fromBytes($this->textFieldPdf(objStm: true));
+
+        $fields = $filler->fields();
+
+        self::assertArrayHasKey('full_name', $fields);
+        self::assertSame('Jane Roe', $fields['full_name']->value);
+    }
+
+    #[Test]
+    public function exposes_the_tu_tooltip(): void
+    {
+        $pdf = PdfDocument::new(compressStreams: false);
+        $page = $pdf->addPage();
+        $page->addFormField('text', 'hint', 0, 0, 100, 20, tooltip: '{"width":"auto","height":140,"left":50,"top":770,"page":1}');
+
+        $filler = ExistingFormFiller::fromBytes($pdf->toBytes());
+        self::assertSame(
+            '{"width":"auto","height":140,"left":50,"top":770,"page":1}',
+            $filler->fields()['hint']->tu,
+        );
+    }
+
+    #[Test]
+    public function decodes_a_non_ascii_tu_tooltip(): void
+    {
+        $pdf = PdfDocument::new(compressStreams: false);
+        $page = $pdf->addPage();
+        $page->addFormField('text', 'hint', 0, 0, 100, 20, tooltip: 'Naam invöeren');
+
+        $filler = ExistingFormFiller::fromBytes($pdf->toBytes());
+        self::assertSame('Naam invöeren', $filler->fields()['hint']->tu);
+    }
+
+    #[Test]
+    public function tu_is_null_when_not_set(): void
+    {
+        $filler = ExistingFormFiller::fromBytes($this->textFieldPdf());
+        self::assertNull($filler->fields()['full_name']->tu);
+    }
+
+    #[Test]
+    public function reads_a_required_readonly_field(): void
+    {
+        $pdf = PdfDocument::new(compressStreams: false);
+        $page = $pdf->addPage();
+        $page->addFormField('text', 'locked', 0, 0, 100, 20, required: true, readOnly: true);
+
+        $filler = ExistingFormFiller::fromBytes($pdf->toBytes());
+        $field = $filler->fields()['locked'];
+
+        self::assertTrue($field->required);
+        self::assertTrue($field->readOnly);
+    }
+
+    /** @return list<string> AS state of every widget of $name, in fixture order */
+    private function widgetStates(ReaderDocument $doc, string $name): array
+    {
+        $node = (new FieldTree())->build($doc)[$name];
+        $states = [];
+        foreach ($node->widgetObjNums as $objNum) {
+            $widget = $doc->deref(new PdfReference($objNum, 0));
+            $as = $widget->get('AS');
+            $states[] = $as instanceof PdfName ? $as->value : '';
+        }
+
+        return $states;
+    }
+
+    #[Test]
+    public function sets_a_text_field_value(): void
+    {
+        $filler = ExistingFormFiller::fromBytes($this->textFieldPdf());
+        $out = $filler->setValue('full_name', 'John Doe')->toBytes();
+
+        $result = ExistingFormFiller::fromBytes($out);
+        self::assertSame('John Doe', $result->fields()['full_name']->value);
+    }
+
+    #[Test]
+    public function fill_sets_need_appearances_on_the_acroform(): void
+    {
+        $filler = ExistingFormFiller::fromBytes($this->textFieldPdf());
+        $out = $filler->setValue('full_name', 'John Doe')->toBytes();
+
+        self::assertStringContainsString('/NeedAppearances true', $out);
+    }
+
+    #[Test]
+    public function sets_a_field_reachable_only_through_parent_inheritance(): void
+    {
+        $path = __DIR__.'/../../fixtures/forms/parent-inherited.pdf';
+        if (!is_file($path)) {
+            self::markTestSkipped('Fixture parent-inherited.pdf not present');
+        }
+
+        $filler = ExistingFormFiller::fromFile($path);
+        $out = $filler->setValue('employer.name', 'Acme Corp')->toBytes();
+
+        $result = ExistingFormFiller::fromBytes($out);
+        self::assertSame('Acme Corp', $result->fields()['employer.name']->value);
+    }
+
+    #[Test]
+    public function flattening_a_parent_inherited_field_removes_the_acroform(): void
+    {
+        $path = __DIR__.'/../../fixtures/forms/parent-inherited.pdf';
+        if (!is_file($path)) {
+            self::markTestSkipped('Fixture parent-inherited.pdf not present');
+        }
+
+        $out = ExistingFormFiller::fromFile($path)
+            ->setValue('employer.name', 'Acme Corp')
+            ->flatten()
+            ->toBytes();
+
+        self::assertSame([], ExistingFormFiller::fromBytes($out)->fields());
+        self::assertStringNotContainsString('/AcroForm', $out);
+    }
+
+    #[Test]
+    public function setting_an_unknown_field_throws(): void
+    {
+        $filler = ExistingFormFiller::fromBytes($this->textFieldPdf());
+        $this->expectException(\InvalidArgumentException::class);
+        $filler->setValue('does_not_exist', 'x');
+    }
+
+    #[Test]
+    public function checks_a_checkbox(): void
+    {
+        $pdf = PdfDocument::new(compressStreams: false);
+        $page = $pdf->addPage();
+        $page->addFormField('checkbox', 'subscribe', 100, 700, 14, 14);
+        $bytes = $pdf->toBytes();
+
+        $out = ExistingFormFiller::fromBytes($bytes)->setValue('subscribe', 'yes')->toBytes();
+
+        $result = ExistingFormFiller::fromBytes($out);
+        self::assertSame('Yes', $result->fields()['subscribe']->value);
+        self::assertSame(['Yes'], $this->widgetStates(ReaderDocument::fromBytes($out), 'subscribe'));
+    }
+
+    #[Test]
+    public function unchecks_a_checkbox(): void
+    {
+        $pdf = PdfDocument::new(compressStreams: false);
+        $page = $pdf->addPage();
+        $page->addFormField('checkbox', 'subscribe', 100, 700, 14, 14, defaultValue: 'on');
+        $bytes = $pdf->toBytes();
+
+        $out = ExistingFormFiller::fromBytes($bytes)->setValue('subscribe', 'no')->toBytes();
+
+        $result = ExistingFormFiller::fromBytes($out);
+        self::assertSame('Off', $result->fields()['subscribe']->value);
+        self::assertSame(['Off'], $this->widgetStates(ReaderDocument::fromBytes($out), 'subscribe'));
+    }
+
+    #[Test]
+    public function selects_one_radio_option_and_unselects_the_rest(): void
+    {
+        $pdf = PdfDocument::new(compressStreams: false);
+        $page = $pdf->addPage();
+        $page->addFormField('radio-group', 'size', 100, 700, 60, 14,
+            options: ['S', 'M', 'L'],
+            radioWidgets: [
+                ['x' => 100, 'y' => 700, 'w' => 14, 'h' => 14],
+                ['x' => 120, 'y' => 700, 'w' => 14, 'h' => 14],
+                ['x' => 140, 'y' => 700, 'w' => 14, 'h' => 14],
+            ],
+        );
+        $bytes = $pdf->toBytes();
+
+        $out = ExistingFormFiller::fromBytes($bytes)->setValue('size', 'M')->toBytes();
+
+        $result = ExistingFormFiller::fromBytes($out);
+        self::assertSame('M', $result->fields()['size']->value);
+        self::assertSame(['Off', 'M', 'Off'], $this->widgetStates(ReaderDocument::fromBytes($out), 'size'));
+    }
+
+    #[Test]
+    public function does_not_mutate_the_original_source_document(): void
+    {
+        $bytes = $this->textFieldPdf();
+        $filler = ExistingFormFiller::fromBytes($bytes);
+        $filler->setValue('full_name', 'Changed')->toBytes();
+
+        // A fresh filler over the same original bytes must still see the
+        // original value — the mutation must not have touched the source.
+        $original = ExistingFormFiller::fromBytes($bytes);
+        self::assertSame('Jane Roe', $original->fields()['full_name']->value);
+    }
+
+    #[Test]
+    public function flattening_all_fields_removes_the_acroform(): void
+    {
+        $filler = ExistingFormFiller::fromBytes($this->textFieldPdf());
+        $out = $filler->setValue('full_name', 'Baked In')->flatten()->toBytes();
+
+        self::assertSame([], ExistingFormFiller::fromBytes($out)->fields());
+        self::assertStringNotContainsString('/AcroForm', $out);
+    }
+
+    private function pageText(ReaderDocument $doc, int $index): string
+    {
+        $contents = $doc->pages()[$index]->dict->get('Contents');
+        $streams = is_array($contents) ? $contents : [$contents];
+        $parts = [];
+        foreach ($streams as $entry) {
+            $stream = $doc->deref($entry);
+            if ($stream instanceof PdfStream) {
+                $parts[] = $doc->streamData($stream);
+            }
+        }
+
+        return implode("\n", $parts);
+    }
+
+    #[Test]
+    public function flattened_value_is_drawn_on_the_page(): void
+    {
+        $filler = ExistingFormFiller::fromBytes($this->textFieldPdf());
+        $out = $filler->setValue('full_name', 'Baked In')->flatten()->toBytes();
+
+        self::assertStringContainsString('(Baked In) Tj', $this->pageText(ReaderDocument::fromBytes($out), 0));
+    }
+
+    #[Test]
+    public function flattening_a_subset_leaves_other_fields_interactive(): void
+    {
+        $pdf = PdfDocument::new(compressStreams: false);
+        $page = $pdf->addPage();
+        $page->addFormField('text', 'first', 100, 700, 200, 20, defaultValue: 'A');
+        $page->addFormField('text', 'second', 100, 650, 200, 20, defaultValue: 'B');
+
+        $out = ExistingFormFiller::fromBytes($pdf->toBytes())
+            ->setValue('first', 'Baked')
+            ->flatten(['first'])
+            ->toBytes();
+
+        $result = ExistingFormFiller::fromBytes($out);
+        $fields = $result->fields();
+        self::assertArrayNotHasKey('first', $fields);
+        self::assertArrayHasKey('second', $fields);
+        self::assertSame('B', $fields['second']->value);
+    }
+
+    #[Test]
+    public function stamps_an_opaque_image_standalone(): void
+    {
+        $pdf = PdfDocument::new(compressStreams: false);
+        $pdf->addPage();
+        $bytes = $pdf->toBytes();
+
+        $out = ExistingFormFiller::fromBytes($bytes)
+            ->stampImage(0, __DIR__ . '/../../fixtures/sample.png', 10, 20, 40, 30)
+            ->toBytes();
+
+        self::assertStringContainsString('/Subtype /Image', $out);
+        self::assertMatchesRegularExpression('@/Stamp\d+ Do@', $out);
+        self::assertStringNotContainsString('/SMask', $out);
+    }
+
+    #[Test]
+    public function stamps_a_transparent_image_with_an_smask(): void
+    {
+        $pdf = PdfDocument::new(compressStreams: false);
+        $pdf->addPage();
+        $bytes = $pdf->toBytes();
+
+        $out = ExistingFormFiller::fromBytes($bytes)
+            ->stampImage(0, __DIR__ . '/../../fixtures/1x1.png', 0, 0, 10, 10)
+            ->toBytes();
+
+        self::assertStringContainsString('/SMask', $out);
+    }
+
+    #[Test]
+    public function composes_fill_flatten_and_stamp(): void
+    {
+        $bytes = $this->textFieldPdf();
+
+        $out = ExistingFormFiller::fromBytes($bytes)
+            ->setValue('full_name', 'Combined')
+            ->flatten()
+            ->stampImage(0, __DIR__ . '/../../fixtures/sample.png', 10, 10, 20, 15)
+            ->toBytes();
+
+        $doc = ReaderDocument::fromBytes($out);
+        self::assertSame(1, $doc->pageCount());
+        self::assertStringContainsString('(Combined) Tj', $this->pageText($doc, 0));
+        self::assertStringContainsString('/Subtype /Image', $out);
+    }
+
+    #[Test]
+    public function rejects_an_out_of_range_page(): void
+    {
+        $pdf = PdfDocument::new(compressStreams: false);
+        $pdf->addPage();
+
+        $this->expectException(\OutOfRangeException::class);
+        ExistingFormFiller::fromBytes($pdf->toBytes())
+            ->stampImage(1, __DIR__ . '/../../fixtures/sample.png', 0, 0, 10, 10);
+    }
+
+    #[Test]
+    public function opens_from_a_file_path(): void
+    {
+        $path = tempnam(sys_get_temp_dir(), 'php-pdf-forms-test-');
+        self::assertNotFalse($path);
+        try {
+            file_put_contents($path, $this->textFieldPdf());
+            $filler = ExistingFormFiller::fromFile($path);
+            self::assertArrayHasKey('full_name', $filler->fields());
+        } finally {
+            unlink($path);
+        }
+    }
+}
diff --git a/tests/fixtures/forms/parent-inherited.pdf b/tests/fixtures/forms/parent-inherited.pdf
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