I love reading on my Kindle, but the screen is just too small and blurry to be useful for PDF files. While it's lacking in customisability, PDF4Kindle solves this problem pretty well by converting PDF files into Amazon's native MOBI format, all while being completely web-based.
Everyone's go-to ebook converting pocket knife, Calibre, can perform the same conversion (and a lot of others) just as well, but if you're on a computer that doesn't have it installed, or just don't want to wade through a ton of menus, PDF4Kindle is a fine alternative. You just upload a PDF file through the site, and download a .mobi a few moments later. I tested it out with a few PDF files, and while it has the normal formatting weirdness you'd expect from an ebook, all the content was there, complete with images and resizable text.
The app isn't magic, so it's not going to turn scanned pages into native text via OCR (try Papercrop if that's what you need), but for any PDF with selectable text, it should do just fine.