Standard PDFs are great, but what happens if you try to open one 50 years from now, and the fonts it used no longer exist?
Enter PDF/A
PDF/A is an ISO-standardized version of the Portable Document Format specialized for use in the archiving and long-term preservation of electronic documents.
It prohibits features ill-suited for long-term archiving, such as font linking (as opposed to font embedding) and encryption. This ensures the document can be reproduced exactly the same way using various software for decades to come.