PDF has everything including the kitchen sink. There’s a fairly esoteric feature called “tagged PDFs” which contain extra markup commands within the page description that map it to a logical structure and alternative textual presentations (e.g. MathML). The problem is that nobody uses it as it’s extremely hard to generate (most software which does text layout discards high level semantic details fairly early on, making it hard to retrofit - pdfTeX for example only recently started inserting space characters between words!). Likewise PDF viewers don’t bother to support it.

Reflowing a tagged pdf is fairly easy... once you have the pdf, which of course you don’t.

It’s a hard problem.