Proof, compliance and data
Digital fingerprint
A digital fingerprint is a short value computed from a file by a hash function, which changes completely if a single byte of the file changes.
Functions such as SHA-256 produce, for a file of any size, a fixed-length string from which the content cannot be recovered. Two identical files have the same fingerprint; it is practically impossible to craft a second file with the same one. It is the basic building block of electronic signatures and timestamping.
Why it matters for fax
Computing the fingerprint of the PDF sent and keeping it with the proof of transmission lets you show later that the file produced is the one that was faxed, not an edited version.