What is a digital signature?

A document to be signed is digitally signed by creating a unique digital fingerprint (hash value) of the document and signing it cryptographically. The signature can be attached directly to the digital document (PDF) and allows the identity of the signer to be established. If the document is changed afterwards, e.g. by changing the content, the digital signature is no longer valid because the digital fingerprint changes. This allows subsequent changes to be detected (integrity).