In the world of eBook file formats, you’ve basically got epub, mobi, lit, and fb2. There are many others, but those seem to be the most widely distributed.
If you’re not buying books from a store tied to your device (Amazon for the Kindle, for example) you generally need to use a third-party tool like Calibre to convert your books to a format best-suited for your device.
However, about 10% of the books you’ll come across are PDFs. PDFs are the absolute worst format to use for ebooks because text is basically inserted in a rigid matrix and isn’t designed to reflow itself based on the device on which it’s being viewed. They’re great for printing, but not so much for reading, particularly on a small screen.
Calibre can convert PDFs to other formats, but it’s a painful process to get it to remove repeating elements like page numbers and headers (which typically appear in opposite locations on alternating pages) and unless you’re a regex whiz, it’s just not worth it.
The appropriately-named Briss solves all that. It’ll let you easily select the text you’d like to keep and discard the rest. It’ll output to a simplified PDF which you can THEN convert with Calibre into something that should work for whatever your ereader-of-choice may be.
It seems like a lot of work to go to, but in the end you’ll have a book that you can take anywhere, instead of one that only looks good on a computer or ipad screen!