Best Tablet Device To Categorize Pdf’s? (can’t Sort By Title, Author, Or Category)?

I tried putting the books I want in different playlists on itunes for use on the iPhone but no luck. All 700+ PDFs are unsortable through Title, Author, and Category and iBooks doesn’t recognize that I put the books in different iTunes playlists.
Can you seperate PDFs by folder on things like kindle paperwhite, kindle fire, Nexus 7, ipad mini?
I don’t want anything big like ipad