.epub is better for books.>>116144594
Nothing. It's terrible, but there's no pressure to replace it. File sizes aren't a problem the way they are for images/audio/video, and it has every single feature it needs (plus a whole lot it doesn't). Hardcore computer weirdos may use PostScript or DjVu instead but it isn't worth the trouble.
Unless you're doing something PDF isn't made for. PDF is for faithfully representing a document with all its formatting and positioning as if you'd printed it out. If exact positioning isn't important then a regular webpage/HTML document or a plain text file or an epub can be better.