Even for such things as contracts, something like PandaDoc or DocuSign is way better than PDF.
I mainly see PDFs used for sending documentation, HR onboarding info, and contracts. None of those uses require immutable files, and they are easy to become obsolete.
Paid e-books could also be done through web readers. But EPUB exists.
For archival purposes, I recognize that there is a use for files, and I listed some options that are all better than PDF.
A PDF reader requires complexity on the order of a web browser. It has enormous amounts of legacy baggage. And it is proprietary and still adding more features regularly. Its existence is akin to that of Flash. It comes from an era where web standards were immature.