Digital signature

A digital signature in a PDF is a block of signed data, made with the signer’s private key over a digest of the document’s bytes. Anyone can check it with the public key in the signer’s certificate.

If a single covered byte changes, the digest no longer matches and the signature fails — which is what makes a digital signature evidence that a document is unchanged. Whether the signer is who they claim depends on who issued the certificate.

In a PDF the signature lives inside the file. A byte range records exactly which bytes it covers, which is everything except the space kept for the signature itself, and the signed data is stored in a CMS (PKCS #7) container.

Two questions are easy to mix up. Integrity asks whether the bytes changed, and the maths answers that for any signature. Trust asks who signed, and that depends on the certificate: one from a certificate authority a viewer already trusts reads as verified, while a self-signed one proves the document is unchanged but says little about the person.

Later changes don’t have to break a signature. A PDF can be saved with an incremental update, which adds new bytes after the signed ones, so a second person can counter-sign and a verifier can still check the first signature against the version it covered.