Our local bus company (rather good) had an "important announcement" as a linked PDF on their web page that was just one paragraph that could have been in HTML. That looks unprofessional
On the other hand, people who want to post old scanned documents or documents that were born in tools like Word or Powerpoint or Publisher can do that efficiently with PDF. For instance, if you go a trade show you will find full-color printed brochures on two sides of 8.5x11 and if you want to distribute that kind of marketing collateral online PDF is a good way to do it.
My favorite use case for PDF is to store PDF documents on my tablet for use in bed, in a hammock, in the lean-to on my hill, in the passenger seat of a car, bus, etc. This is good for "e-book"(s) as well as it is for personal documents.
For years I have always made a "go folder" when I travel that has everything I need to get from home to the hotel and back. I still make one, but I also store bundles of PDF documents on my tablet to support this. It's like I can carry all the books in my house on my person, which is pretty good.
How else would you place documents on your website that you have made available for download or reading on-site? You need those documents to be in some cheap, widely available format.
In today's world, that format is .PDF. (A few decades back, that might have been Microsoft's Word .DOC, and earlier still it would have been plain old .TXT.)