> I think I always assumed if someone was reading on a smaller screen, an epub would suffice
If it helps to understand why someone would stick to PDFs: I want to both preserve my ebooks over the long term (decades) and annotate them - comments, highlighting, etc. ePub's format is less stable for the long term, but most of all it lacks standardized annotation (afaict, after some detailed research). Also, if I annotate a book on my computer, I want the same copy on my small screen.
If it helps to understand why someone would stick to PDFs: I want to both preserve my ebooks over the long term (decades) and annotate them - comments, highlighting, etc. ePub's format is less stable for the long term, but most of all it lacks standardized annotation (afaict, after some detailed research). Also, if I annotate a book on my computer, I want the same copy on my small screen.
I hope that's helpful!