Verify PDF Signature

For every digital signature in the file: whether the signed content is exactly what was signed, whether the signature matches the signer's certificate, who the certificate names and who issued it, when the document was signed - by a timestamp authority or only by the signer's own clock - and whether anything was added to the file afterwards. A certification signature also shows which changes it allows.

Yes, against the EU trusted lists - every member state's qualified trust service providers, with the dates their services were granted - and the U.S. Federal PKI root. The lists come as a snapshot with the site, so nothing is sent anywhere. A signature made with a qualified certificate on a signature device, from a provider that was granted at the signing time, is shown as a qualified electronic signature or seal. Revocation is read from the data the signer stored in the file (OCSP responses, CRLs); the check never contacts the issuer. Providers on other lists, such as Adobe's, and revocation without stored data are not checked here.

A PDF can be added to after it is signed: a second signature, a filled-in field or a comment is appended to the end of the file, and the signed part stays as it was. If a later signature covers the additions, the report says so. If nothing covers them, the signed version is still intact, but what you see may not be what was signed. A change inside the signed part breaks the signature. For such a document you can save the version the signature covers, or open it beside the current one in Compare PDFs to see what changed.

No. The check runs in your browser, on your device: the file, its signatures and the certificates never leave it. Nothing is produced, so it doesn't count against the daily limit either.