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 applied directly to the digital document (PDF) and allows the identity of the signer to be established. If the document is subsequently modified, e.g. by changing the content, the digital signature is no longer valid because the digital fingerprint changes. This means that subsequent changes can be detected (integrity).