I just checked the ticket correspondence and the statement in there seems quite clear:
ALL PDFS are not allowed on free hosting anymore.
Which tells me that 1) PDFs are indeed blanket banned and 2) this is probably a fairly new rule.
I didn’t know that this was the case. Although I have seem quite a few bad sites using PDFs to do nasty stuff. And I can imagine PDF inspection isn’t easy enough that scanning the contents of PDFs is feasible.