The Shifted Librarian does a much better job than I did of expressing why PDF is a dead end format. If you archive something in PDF, what are the chances you’ll be able to get at the underlying content 50 years from now? Roughly zero. XML, on the other hand, can always be parsed.
This archival failure is a major risk for most digital formats and will become an ongoing crisis, sort of like Y2K spread over the next fifty years.