Welcome to MobileRead!
I wish I could definitively answer your questions. I THINK web books are any books that are published on the Internet. Ebooks are generally books that exist in a digital format of some kind like pdf, lit, lrf, etc.
There is a forum here strictly dedicated to ebook format conversions. I haven't dealt with pdf files, but I know many of our experts here have. If you go to the top of the page and click on Forums you'll be able to look there for more help. Good luck.
Enjoy!
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