Proof, compliance and data
PDF/A
PDF/A is the standardised version of PDF for long-term archiving: the file contains everything it needs to be displayed identically decades from now.
Defined by ISO 19005, it requires fonts to be embedded and forbids anything that would make display depend on external or unpredictable elements: external content, scripts, encryption. Several versions coexist, from PDF/A-1 to PDF/A-4. A PDF/A remains a PDF that any software can read.
Why it matters for fax
Archiving a sent or received fax as PDF/A ensures you can read it unchanged on the day you need to produce it. The format freezes the appearance; combined with a fingerprint and a timestamp, it also lets you show the file has not changed.