Well, this was a total lifesaver when I figured it out. I used to be so frustrated emailing books to my Kindle and having the formatting go all weird. The dedicated 'Send to Kindle' web page you find on Amazon's site is just way more reliable. You drag and drop your file—EPUB, PDF, whatever—into the browser window, pick the device you want it on from the list, and hit send. No messing with email attachments or worrying about the 'convert' option. The main trick is making sure you're logged into the same Amazon account that your Kindle is registered to. I've sent over a dozen fan-translated webnovels this way, and they show up in my library cleanly every time. It feels like you're sideloading but without any cables.
One thing that tripped me up at first was file size. There's a limit, something like 200 MB, which is huge for text but might matter for PDFs with lots of images. Also, the title metadata from the file doesn't always transfer perfectly; sometimes the book shows up with the filename as the title. Minor quibble, though. It's become my standard method for getting stuff from Project Gutenberg or author review copies onto my Paperwhite.