How can you check that a document has been validly signed and has not been subsequently modified?

A PDF reader (such as Adobe Acrobat Reader) can verify the validity of an electronic signature. When the document is opened, the PDF reader automatically performs two operations:

• It calculates the document’s fingerprint to see if it matches the fingerprint of the document that was signed;
• It checks the digital certificate to ensure its validity (therefore the authentication of the signatories).

If the reader recognizes one of these elements as invalid, a message will warn the user that the electronic signature is invalid. This removes the need to initial the document on each page.