Your viewer says 'the document has been modified since this signature was applied'. The file looks normal. Is something wrong?
What's actually going wrong
The cryptographic signature commits to a specific file state. Any change since signing — even one byte — invalidates the commitment, and the viewer reports 'modified'.
Causes can be benign (someone saved the file, adding metadata without changing content) or concerning (someone deliberately altered content). The warning doesn't distinguish; it just reports the math doesn't match.
The quick fix
Investigate. Most viewers show what changed — Acrobat shows page-level revision history for signed PDFs. Compare to the signed version if you have it.
If changes are benign and you trust the source, the warning is informational rather than alarming. If you don't trust the source, treat the document as untrusted.
For documents that need to remain provably untouched, never edit or save them after signing. Treat signed PDFs as read-only.
If that didn't work
If you need a signed PDF without the modification warning, get a fresh copy from the signer. They sign a clean version; you keep it unmodified.
For your own signed documents that got modified, re-sign with Flint — fresh signature on current state. Note this is a new signature, not a restoration of the old.
Prevent it next time
Treat signed PDFs as final. Don't save them after opening. Don't run them through any processing tool. Store and view only.
FAQ
Is a modification warning always serious?
Not always. Some viewers add metadata on save that triggers the warning without changing content. But the warning is the correct response — the file isn't byte-identical to its signed state.
Can I see what was modified?
Acrobat and other signature-capable viewers show page-level revision comparisons. Other viewers may not.
Does the warning mean the document is invalid?
Cryptographically the signature no longer proves untouched-ness. Legally it varies — courts may consider context. Practically, treat the document with appropriate scepticism.
Can I fix the warning?
No, the warning is permanent for modified-since-signing files. You can re-sign for fresh validity, but the original signature stays invalidated.
Modified warnings need understanding, not bypassing. For fresh trust, re-sign with Flint.