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fix: ensure PDF form appearance streams have required /Subtype /Form entry (#2328)

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When flattening PDF forms, some appearance streams lack the required
/Subtype /Form dictionary entry needed when used as XObjects. This
causes
corruption in Adobe Reader which fails to render these flattened fields.

Per PDF spec, Form XObject streams require:
- /Subtype /Form (required)
- /FormType 1 (optional)

The normalizeAppearanceStream function ensures these entries exist
before
adding appearance streams as XObjects to the page content stream.

Fixes rendering issues where flattened fields don't display in PDF
viewers.
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Lucas Smith 2025-12-16 16:00:11 +11:00 committed by GitHub
parent c5c87e3fd1
commit f129968968
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1 changed files with 35 additions and 0 deletions
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35
packages/lib/server-only/pdf/flatten-form.ts
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@ -4,8 +4,10 @@ import {
PDFDict,
type PDFDocument,
PDFName,
PDFNumber,
PDFRadioGroup,
PDFRef,
PDFStream,
drawObject,
popGraphicsState,
pushGraphicsState,
@ -103,6 +105,36 @@ const getAppearanceRefForWidget = (field: PDFField, widget: PDFWidgetAnnotation)
}
};
/**
* Ensures that an appearance stream has the required dictionary entries to be
* used as a Form XObject. Some PDFs have appearance streams that are missing
* the /Subtype /Form entry, which causes Adobe Reader to fail to render them.
*
* Per PDF spec, a Form XObject stream requires:
* - /Subtype /Form (required)
* - /BBox (required, but should already exist for appearance streams)
* - /FormType 1 (optional, defaults to 1)
*/
const normalizeAppearanceStream = (document: PDFDocument, appearanceRef: PDFRef) => {
const appearanceStream = document.context.lookup(appearanceRef);
if (!(appearanceStream instanceof PDFStream)) {
return;
}
const dict = appearanceStream.dict;
// Ensure /Subtype /Form is set (required for XObject Form)
if (!dict.has(PDFName.of('Subtype'))) {
dict.set(PDFName.of('Subtype'), PDFName.of('Form'));
}
// Ensure /FormType is set (optional, but good practice)
if (!dict.has(PDFName.of('FormType'))) {
dict.set(PDFName.of('FormType'), PDFNumber.of(1));
}
};
const flattenWidget = (document: PDFDocument, field: PDFField, widget: PDFWidgetAnnotation) => {
try {
const page = getPageForWidget(document, widget);
@ -117,6 +149,9 @@ const flattenWidget = (document: PDFDocument, field: PDFField, widget: PDFWidget
return;
}
// Ensure the appearance stream has required XObject Form dictionary entries
normalizeAppearanceStream(document, appearanceRef);
const xObjectKey = page.node.newXObject('FlatWidget', appearanceRef);
const rectangle = widget.getRectangle();