What is the difference between PDF and ePUB?
This answer comes via New York Public Library:
"It helps to think of a PDF as a snapshot of a page in a physical book or journal. Charts, images, and diagrams will appear the same way they appear in the printed book. This is great for picture books as well as others where the design is key. If you enlarge the page, you are zooming in to a section; this sometimes makes it not a practical option for people with low vision.
With EPUB, the emphasis is on the text itself. You can resize the text in either direction, to make short pages with large text or long pages with tiny text (this is referred to as "reflow")."
Also, here is a link that provides a good explanation and a handy chart of the differences between the two: http://blog.techw.com/?p=287.