Signatures
How to fill or sign a PDF without invalidating the signature that’s already there
Someone signed the PDF before you. You need to fill your part and sign too. Here is why that so often breaks the first signature, and how to avoid it with any app.
Only make changes the signature allows (usually filling form fields, adding comments and adding another signature) and save with an app that appends changes to the end of the file instead of rewriting it. Don’t print to PDF, flatten, compress, convert or “save as optimized”. Then open the result in a signature validator and check that the first signature still shows as valid.
Why a signed PDF breaks so easily
A digital signature in a PDF doesn’t protect what the page looks like. It protects a list of exact byte ranges in the file. When the document is signed, the software calculates a fingerprint (a hash) of those bytes and seals it with the signer’s certificate. A validator later recalculates the fingerprint. If even one byte inside the signed range is different, the result doesn’t match and the signature is reported as invalid.
That is why a document can look identical and still fail. Many apps save by rebuilding the whole file from scratch: objects get renumbered, streams get recompressed, and the byte ranges the signature points to no longer contain what was signed.
In our own test on November 4, 2026, we filled one form field in a signed PDF and saved it in Preview on macOS 26.5. Preview rewrote the file, the signature stopped matching its bytes, and the attachment, layer and page labels inside the file were gone. It showed no warning. Another app we tested, PDFgear, refused to overwrite the signed file and saved a copy, but the copy had no signature at all.
The safe way: incremental updates
The PDF standard has a built-in answer. Instead of rewriting, a program can add an incremental update: the changed objects, a new cross-reference table and a new trailer are appended after the end of the original file. The original bytes stay exactly where they were, so the first signature keeps covering what it covered. This is defined in ISO 32000-1, section 7.5.6, and it’s how a second signature is normally added to a PDF.
A validator then sees two things: the signed revision, still intact, and a later revision with your changes. Whether that later revision is acceptable depends on what the signer allowed.
Which changes a signature allows
A signer, or the tool that prepared the form, can set permissions. The PDF standard defines three levels for a certifying signature (the “DocMDP” permissions in ISO 32000-1, section 12.8.2.2):
| Permission | What you can still do |
|---|---|
| 1 · No changes | Nothing. Any change invalidates the certification. |
| 2 · Form filling and signing | Fill form fields, add signatures in existing signature fields. |
| 3 · Also annotations | The above, plus comments, highlights and notes. |
Specific fields can also be locked after a signature (“field MDP”), which is common when the first signer’s name and date shouldn’t be edited. Ordinary approval signatures, with no permissions set, are usually treated like level 2 or 3 by validators: filling, commenting and signing are accepted, other changes are flagged.
What is never fine after signing: editing text or images on the page, deleting or inserting pages, flattening fields, running OCR, or converting the file. Those change the content the signer approved.
Step by step, with any app
- Check what was signed. Open the signature panel and look for “certified” and its permissions. If it says no changes are allowed, ask the sender for an unsigned copy or a new signature field.
- Fill the existing fields. Type into the form’s own fields rather than placing text boxes over the page. Text boxes are annotations, which level 2 doesn’t allow.
- Sign in an empty signature field if there is one. Adding a new signature field after a certification can count as a disallowed change.
- Use plain Save. Avoid Print to PDF, Export, Reduce File Size, Optimize, Save as PDF/A and any “flatten” option. Each one rewrites the file.
- Check the result. Open the saved file in a validator, such as the signature panel of Adobe Acrobat Reader on a computer. The first signature should read as valid, with your changes listed as permitted changes after signing.
- Keep the original until the other side confirms they can validate both signatures.
One case no app can fix: if the first signer ticked “Lock document after signing” in Acrobat, the file is locked for everyone, the signer included. Adobe’s advice is to ask the owner for an unsigned copy, or recreate the document.
Mistakes that break signatures without you noticing
- Saving from an app that rewrites the file, often the default viewer on your system.
- Making the file smaller before emailing it. Compression always rewrites.
- Scanning a printed copy and signing that. You get a picture of a signature, not a signature.
- Adding your signature as a pasted image over the page of a certified document.
- Letting a cloud service “prepare” the document, which can flatten or regenerate it.
Keepsign always saves by appending, on iPhone, iPad and Mac. Before saving, it compares your changes with each signature’s permissions and locked fields. If something would invalidate a signature, it tells you whose and why, before writing. After saving, it reopens the file, verifies every signature again and gives you a receipt: “Your signature stays valid”, with each signature before and after. Filling, signing and the receipt are free. More about signing without breaking.
Sources
- ISO 32000-1:2008, Document management — Portable document format — Part 1: PDF 1.7, sections 7.5.6 (Incremental updates), 12.8 (Digital signatures) and 12.8.2.2 (DocMDP). Public copy published by Adobe: opensource.adobe.com/dc-acrobat-sdk-docs/pdfstandards/PDF32000_2008.pdf
- Keepsign test of November 4, 2026, macOS 26.5.2: one signed PDF edited and saved in Preview, PDFgear 2.29, PDF Expert 3.13.5 and UPDF 2.5.7; signatures checked with OpenSSL against the signed byte ranges. Details on the comparison page.
- Adobe, “Validate digital signatures”, helpx.adobe.com/acrobat/desktop/e-sign-documents/manage-digital-signatures/validate-digital-sign.html, and “Permissions and limitations of signed PDFs”, helpx.adobe.com/acrobat/desktop/e-sign-documents/learn-about-signatures/signed-pdf-limitations.html, read November 5, 2026.