PDF/A is an open standard, but it's pretty rare to see a PDF document in the wild that is actually PDF/A compliant. People generally just use the built-in PDF e

PDF/A is an open standard, but it's pretty rare to see a PDF document in the wild that is actually PDF/A compliant. People generally just use the built-in PDF export in some word processor (usually no PDF/A option at all) or use Acrobat (doesn't do PDF/A by default). And non-PDF/A documents can have a laundry list of odd closed features that may or may not be supported by various readers. In general the landscape of PDF compatibility has improved a lot, but it's still a lot worse than HTML.