EPUB files are one of the most popular ebook formats. They have an advantage over PDF files (discussed below) in that they will reformat their content to the size of your screen. This means that if I view the book or document on a computer screen it will have more words per line, more lines per page, thus it may only have 10 pages. On a smart phone, that same document will have fewer words per line, fewer lines per page and it may have 100 pages. This is an advantage when reading on a smart phone as it means you can increase/decrease the text size to suit your eyes, and only have to turn pages, not scroll horizontally. EPUB files are viewable on almost all devices.
PDF files are one of the most popular format for sharing information. Their primary benefit is that they layout stays the same across all devices. This means that if the author of the book or document want words or pictures to appear in a specific place on a specific page, they will and all those viewing it will see it the way it was intended. This means that on a smart phone you’ll need to scroll horizontally and vertically to see the entire page. PDF files are viewable on almost all devices.
So, to answer the original question, which is best?
If you’re using a smart phone or tablet, I’d tend toward using EPUB files. If your primary use is on a computer/laptop and tablet then I’d tend toward a PDF.