You open the signed PDF. A yellow warning bar at the top: 'At least one signature has problems.' Or red: 'Signature is invalid.'
This is a real signal worth investigating before treating the document as authentic.
What's actually going wrong
File modified after signing. Any edit (even a save in some viewers) breaks the cryptographic commitment. Certificate not trusted. The signer's certificate authority isn't in your viewer's trust store. Certificate expired or revoked. Signature was valid when made but the certificate later became invalid. Self-signed certificate. Common in personal signatures; viewers don't trust self-signed by default.
Click the warning to see which case applies. Acrobat and most viewers show the specific reason.
The quick fix
File modified: The signature is broken. You can't restore a broken signature. If you trust the source, the document content may still be authentic; if you don't, treat it as untrusted.
Trust issues: If you know the signer is legitimate, add their certificate to your trust store. Settings vary by viewer.
Need to re-sign: Use Flint's sign tool to apply a fresh signature. Note this is your signature, not the original signer's.
If that didn't work
If the signature is critical (legal document), ask the signer to re-sign and resend. Repaired signatures don't exist — broken means broken.
For batch documents where signature validity matters, validate before storage. Catch issues early.
Prevent it next time
Don't edit signed PDFs. Once signed, treat them as finished. If changes are needed, ask the signer for an unsigned working copy and re-sign at the end.
FAQ
Can I fix an invalid signature?
If the file was modified, no — the signature is broken. If it's a trust issue, configure your viewer's trust store. If the signer needs to re-sign, that's their action.
Does saving a signed PDF invalidate the signature?
In some viewers yes, especially if the save rewrites the file structure. Open signed PDFs read-only; don't save unless necessary.
What's the difference between signature and certificate problems?
Signature problems: the cryptographic check fails (file was modified). Certificate problems: the signature itself verifies but the issuer isn't trusted by your viewer.
Are self-signed PDFs legitimate?
They can be — for casual use. For legal or business contexts, certificate-authority-issued signatures provide third-party verification that self-signed doesn't.
Invalid signatures need understanding, not bypassing. Use Flint for fresh signatures when re-signing is the right move.