Good question! It depends on how you intend to read your book.
PDF files are great for reading on your computer, and can also work on your tablet if you have a large enough screen. PDF files have their limits, though. The pages are fixed in size, so if you zoom in to far, you’re going to be doing a lot of scrolling back and forth, and nobody wants to do that. As the world continues to move farther into the realm of mobile tech, the PDF file isn’t for everybody.
EPUB is the format of choice if you’re using any eReader, most notably the Amazon Kindle (whether the traditional black-and-white reader or the Fire tablet). This is also the file you want if you’re going to be using the Kindle app on any of your devices. Amazon is without question the leader in eBooks, and we endorse the Kindle family of products without hesitation. If you’re looking for a standalone device for reading, the Kindle Paperwhite is the way to go. You just can’t beat the weeks-long battery life with any cell phone or tablet, and it’s far cheaper than the Kindle Oasis.
EPUB files are compatible with iBooks, Barnes and Nobles’ Nook, Kobo, and almost everything else.