PDF signature invalid: "document has been altered or corrupted since it was signed"
Last reviewed 2026-09-19
A digital signature in a PDF covers exact bytes of the file. If anything within that range changes, even by re-saving in another program, the signature no longer matches and viewers warn that the document was altered. A different warning, "validity is unknown", usually means the signer's certificate isn't trusted on your computer, not that the file changed.
Messages you might see
- “The document has been altered or corrupted since the signature was applied”
- “At least one signature is invalid”
- “Signature validity is unknown”
Why it happens
- The file was edited, compressed, flattened or re-saved after it was signed.
- An email gateway or upload tool modified the PDF.
- The file was cut off or damaged after signing.
- Later signatures or form filling were added in a way that is not allowed by the first signature.
Try this first (free, no tool needed)
- Ask the sender for the original file straight from the signing service.
- Open it in Adobe Acrobat and use the signature panel for the full details, including certificate trust.
- Do not open and re-save the signed PDF in another editor.
If that didn't work
These tools run in your browser. Your file is read and repaired on your own device and is never uploaded.
Broken Digital Signature PDF Validator
Reads each digital signature, recomputes the fingerprint of the signed bytes and compares it with the signed value: it reports intact or changed, who signed, when, the stated reason, and whether content was added after signing. This checks the integrity of the signed content only. It does not check that the signer's certificate is trusted, not revoked or not expired, so it is not a legal validation.
Open Broken Digital Signature PDF ValidatorCommon questions
If it says intact, is the signature legally valid?
Not necessarily. It shows the signed content is unchanged. Trust in the signer's certificate needs a proper validator such as Acrobat or a validation service.
Is my file uploaded anywhere?
No. The tool runs as a program inside your own browser, so the file is read and repaired on your device and never sent to a server.