For any document in the form of PDF files, this is fairly trivially done using a certified PDF signed by a member of the Adobe Approved Trust List. Other data c

For any document in the form of PDF files, this is fairly trivially done using a certified PDF signed by a member of the Adobe Approved Trust List. Other data could be signed by PGP from an issuer that publicizes its public key. In practically every use case there exist trusted parties, obviating the value of decentralized certification.