You can connect your Kindle to a USB port on your PC, transfer PDFs of whatever type across and then read them. As mentioned above, Project Gutenburg ( http://www.gutenberg.org/
) has enough free stuff to keep you going for a few centuries.
I'd really recommend paying a little extra to get the Kindle version with the keypad at the bottom. This makes it much
easier to navigate reference books and search for things to download. If you get the 3G version, you can also use it for emergency internet browsing and email on the move in many countries. It's far from perfect for that, but just about workable.