How to sign a PDF

An electronic signature can be a drawn mark, a typed name or an image placed on a page. It may be useful evidence of intent in a particular workflow, but the mark alone does not cryptographically bind the signer to the document bytes or prove who placed it.

A digital signature uses a private key and certificate to sign a defined byte range. A capable PDF viewer can then check whether the covered bytes are unchanged and whether the certificate identity chains to a trust anchor. Those are separate integrity and identity questions, and a visible appearance does not answer either one.

For a contract, filing or other consequential document, use signing software and a certificate workflow appropriate to the jurisdiction and counterparty. Do not treat a picture of a signature, a flattened mark or an inspector report as equivalent.

PDF Signature Inspector reports whether signature field objects are present, including fields that may still be empty placeholders. It is an inspector, not a trust validator: it cannot establish that a certificate is trusted, that revocation evidence is current, or that a legal requirement is satisfied.

Flatten PDF Form turns existing form appearances into page content. Flattening is not signing, does not create tamper evidence and is not tamper-proof. It is a one-way presentation change, so keep the original and verify the result visually.

Watermark PDF adds a visible label such as DRAFT or CONFIDENTIAL. A watermark communicates status; it does not identify a signer, encrypt a file or prevent edits.

Any edit, merge, flattening operation or other rewrite can invalidate a cryptographic signature because the signed bytes change. Keep the original signed file intact and make a separate reading or presentation copy when needed.

If a viewer reports a signature as invalid or the signer as untrusted, an inspector can help you understand what is present, but it cannot turn that result into a valid signature. Follow the signer, certificate provider or records process responsible for the document.