Pdfs are terrible for letter style docs on phones (reflowable text is better, say in emails), but they are great as slides. Certainly easier to distribute or display than proprietary PPT editors.
Reflowable text is fine for conveying information on a tiny screen, but that’s about it.
When your screen is bigger than a page, reflowing the text is almost never what you want because long lines are hard to read. Thus even though computer screens aren’t limited to the size of a sheet of paper, you want to wrap your lines at paper-width (or better, book width) anyway.
Note that programmers typically define an effective page width for source code by enforcing an 80-100 column limit.
I wish pdf would die. It's a left over vestage of a paper world. It's also left over from a non-connected, non-international world A4 vs US Letter.
PDF really has no place in this smartphone first world