I don't know about the old PDFs of RFCs scanned from paper, but at least the ones being generated now are not just "PDF" but "PDF/A" (at least that's what RFC 7995 called for).
PDF/A is an ISO-standardized version of the Portable Document Format (PDF) specialized for use in the archiving and long-term preservation of electronic documents. PDF/A differs from PDF by prohibiting features unsuitable for long-term archiving, such as font linking (as opposed to font embedding) and encryption.[1] The ISO requirements for PDF/A file viewers include color management guidelines, support for embedded fonts, and a user interface for reading embedded annotations.
Of course, you can declare anything to be there for "the long term" and lose because of disuse, but the widespread adoption of PDF/A as a long term archival format increases the likelihood of interpretability.
On the other hand, there is no SVG/A (the analog of PDF/A but for SVG. This means that the likelihood that, say, 50 years from now, you won't be able to find a compatible renderer for IETF based RFCs in HTML+SVG is (IMO) greater.
I'm a fan of preserving software through emulation (engaged in preserving some old software from the 80s) but I think to be careful it might be important to archive the Tools implementations of various versions of the RFC XML formatted. Also when regenerating the text, PDF, HTML derived forms, being careful to archive the previous renditions (as a safety measure).
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