Depends on your use case.
Ever notice that a Kindle or your local library's online books don't use PDFs? (You do know your local library has digital versions of most books don't you? For free...) Their are formats like ePub and Mobi that are designed for reading on variable sized screens. Ever notice how much it sucks to try to read a two column layout of most PDFs on a small screen or even any landscape oriented screen? ePub and Mobi don't have two columns, they have the text (and inline images) just inline images and let the reader format them for column width, text size, text color, background color, etc.
(edit: and the reason they come as PDFs is because with a couple of mouse clicks you can convert a print ready book to PDF and because (wrongly so) most people think PDFs are 'secure' and can't be altered or text scraped once created. PDFs (for reasons) are ubiquitous, therefore people think its what they should expect or want.)
That's just for reading. If you actually want to use the content during game play, then the VTT of your choice might have a better format that can do all the things I mentioned before.