What's the difference between the PDF format, ePUB format and MOBI format?
A PDF format is an electronic copy of the printed book, and the placement of items on the page are static and will never move. This makes the PDF format ideal for printing, and possibly viewing on your computer monitor. A PDF format is more difficult to read on small readers where the page size of the reader is not as large as the original book because the PDF doesn't scale easily to be viewed on different sizes.
An ePUB format, on the other hand, is a long stream of text and images that can be re-flowed easily to different sizes of viewing. Having text that re-flows makes increasing font sizes while maintaining readability much easier. This makes the ePUB format ideal for electronic reading devices such as the iPad, Sony Reader, Kindle, Kobo, and others. The disadvantage of the ePUB format is that it is not as pleasant to read if the ePUB version is printed.
A MOBI format is a Kindle-friendly digital book format. Your Kindle will not read the ePUB format. This format can only be read on a Kindle, unless you have some other device that specifically indicates you can use the .MOBI format.