You signed the contract, then noticed a typo on page 4 and corrected it. Now the signature shows as invalid. That's not a bug — that's exactly what's supposed to happen.
The cryptographic reason
Signing computes a hash over the document at the moment of signing. The signature binds the signer to that exact hash. Edit any byte of the document and the new hash differs from the signed hash. The signature is no longer over the document you have — it's over a document that no longer exists.
Viewers detect the mismatch and show the signature as invalid. They're doing their job.
Why this is a feature
If signatures survived edits, anyone could sign a contract and later 'correct' clauses to their benefit. The whole point of signing is to freeze the document. Breaking signatures on edit is the mechanism that makes signing meaningful.
It's why every serious signing workflow puts signing as the very last step. Get the document right first; sign once everyone has agreed; never edit afterwards.
If you need to change a signed document
Don't edit. Issue a new version. The old signed PDF remains evidence of what was agreed on day one; the new version goes through signing again with all parties.
For minor typos in a signed contract, the standard fix is a side letter — a short separate document the parties sign confirming the correction. The original signature stays intact.
FAQ
Can I add a comment to a signed PDF without breaking the signature?
Some viewers allow annotations as 'incremental updates' that don't break the existing signature. The annotation appears but the original signed content is unchanged. Verify in your specific viewer.
What about filling form fields after signing?
If the signature explicitly permits form fills (rare), it can survive. Most signed PDFs treat any change as invalidating.
Can I re-sign after editing?
Yes — apply a new signature over the edited version. The old signature is now historical evidence of what was signed before the edit.
Does cropping or rotating pages break the signature?
Yes. Any structural change to the document — including page rotation in a way that's saved into the file — alters the hash.
Sign last, sign once. Use Flint at the end of your workflow, not the middle.