Flattening PDFs: Forms, Annotations and Signatures

"Flatten the PDF" is advice people give for forms that look wrong, comments that disappear, and documents that must not be edited. It is useful, but it changes the file in ways that are easy to misunderstand. This guide explains what is flattened, what is not, and what happens to signatures.

What a PDF is made of

A PDF page has content: text, images and drawings. On top of that it can have annotations, separate objects drawn by the viewer: comments, highlights, stamps, ink, shapes, links, and the widgets of interactive form fields. Each annotation has a rectangle on the page and, usually, an appearance stream that says how to draw it.

What flattening does

Flattening draws the appearance of those objects into the page itself and removes the objects. After that a filled text field is just text on the page, a ticked box is a drawn tick, and a highlight is a coloured shape. Nothing can be edited as a field or moved as a comment any more, and every viewer shows the same thing. The Flatten PDF tool does this in your browser, reports how many fields and annotations it handled, and keeps links as links.

What it does not do

• It is not security. The text and data are still in the file and can be copied. Flattening does not encrypt or redact anything. To hide data, remove it; see PDF Personal Data Scanner for finding it and ID Copy Watermark & Redact for image-based redaction.
• It does not draw what has no appearance. An annotation without an appearance stream, for example a note some viewers draw themselves as an icon, cannot be flattened, so a tool leaves it and says so.
• It cannot be undone. Keep an editable original.

Digital signatures

A digital signature in a PDF covers a range of the file\'s bytes. Any change, including flattening, breaks that coverage, so the signature becomes invalid, and signature fields disappear. Some PDFs allow certain changes after signing (form filling, adding comments) through "certification" rules, but rewriting the whole file to flatten it is not one of them. If a valid signature matters, do not flatten the signed file: flatten a copy before it is signed, or sign after. To see what a PDF\'s signatures say before changing anything, use the PAdES PDF Signature Inspector; its guide PAdES and eIDAS signatures explained covers what that inspection can and cannot show.

Forms: a few particular cases

• Text fields. Flattened with the text they show. If a value cannot be drawn with a built-in font (for example non-Latin text without stored appearance), the stored appearance is used.
• Check boxes and radio buttons. Flattened as the state they are in; each state has its own appearance.
• Dynamic (XFA) forms. Only the AcroForm appearance viewers show can be flattened; the XFA description is dropped.
• Scripts. Calculations and validation stop applying, since there are no fields left.

A sensible routine

1. Save an editable copy.
2. Check what the file contains with PDF Information and the analysis step of the flatten tool: field types, annotations without appearance, signatures.
3. Flatten, then compare each page with the original, especially checkboxes, stamps and highlights.
4. If you also need to remove hidden descriptive data, use the PDF Metadata Viewer & Remover.

If you only need to change text on a page, the PDF Text Editor works on the page content itself.