I've yet to view a scanned PDF that was very readable on a reader. Unless you can use OCR software, most of which does a lousy job of converting scanned images to reflowable text, you will generally have crap on your screen. If you can use OCR software and create reflowable text, and then if you spend hours editing to correct the copious mistakes of the OCR software and to format the book so that images and tables appear where and as they should, you might wind up with a decent ePub. It is a lot of work that might take as long as reading the doc in printed format. Notebook paper sized PDF files are not designed to be viewed on a book reader. They are designed to be printed. They really don't even work that well on large computer monitors.

Bottomline: If you can create truly reflowable text from the scanned doc, then with some work you can create a very readable ebook. If your scanned doc looks like a copy of a copy of a copy of a copy of a copy, that is rather fuzzy and difficult to read even when printed, then you probably won't be able to create a usable ebook.