I've read a lot of library books. Kobo is great for ePubs (except for some rare badly-formatted ones; I've never seen that myself).
You're right that PDFs can be difficult to read on the small screen, as panning is required and it's not a great reading experience. You would have to remove DRM to reformat PDF and then it's not guaranteed to work. (I've never done this as I avoid PDF)
Philadelphia library accepts non-resident members: Here is a very long thread with lots of commentary including how to apply:
Also, be aware that there can be long waiting lists for eBooks. Just like paper books, but it's harder to return books early so many people keep them for the full checkout limit.