PDF/A is a small family of sub-formats to PDFs which amount to the accessible subset of PDF + some requirements.
PDF/A and PDF/UA are mandated by a variety of governments for PDF distribution. They're very suitable to archival and are accessible (screenreaders can work with them, their text can be cleanly copied, etc).
That might be true, but these ones are not. I can't efficiently read them on my smartphone and have to scroll around a lot. They also show two pages on one, probably to be better printable for some marketing material, but I would never want to print a PDF that's mostly black, because that would overwhelm my printer. I'm also not near a printer right now. This is clearly inaccessible to me.
HTML, on the other side, can be easily made readable on any device. Even if the author didn't care, through readable mode in my browser.
PDFs are often the BEST way to read something. I prefer PDF text books over any other format, because (not very good) computer algorithms laying out text is inferior to proper professional layout.
PDF/A isn't necessarily accessible. First, PDF is very hard to impossible to scale for people who need big writing, due to reflowing being unavailable or not working properly. Text in PDF/A can still be a big image, e.g. in a scan for archival purposes, if you are "lucky" there is an OCR overlay, but then your screenreader reads OCR'd text which is hit-or-miss. And text ordering is supposed to be "reading order" in PDF/A, but many tools, especially DTP tools make a mess of it and jumble up the (screenreader-visible) order of text in textboxes, columns and sometimes even paragraphs.
IME PDF/A is better than nothing, but far worse than plain HTML for accessibility..
Scanned images are totally PDF/A-compliant. The standard is just about reproducible archiving, nothing else. Bitmaps are perfectly reproducible and pass every compliance check. That PDF/A is more accessible than any old PDF is just a side-effect.
This is their strength: You know a PDF will not shift its presentation due to device changes. PDFs from when the PDF was created still work right today. And they still look the same.
https://en.wikipedia.org/wiki/PDF/A
PDF/A is a small family of sub-formats to PDFs which amount to the accessible subset of PDF + some requirements.
PDF/A and PDF/UA are mandated by a variety of governments for PDF distribution. They're very suitable to archival and are accessible (screenreaders can work with them, their text can be cleanly copied, etc).