Yup - it'll take PDFs (and there's a way to email them directly to your Kindle/Kindle acct. I forget how, exactly, but you can find it online very easily — you essentially have an email address associated w. your kindle, and you can just email the document, as an attachment, to it). PDFs are often somewhat messier/don't work as well, especially with all the features, but if you use Calibre (free app/software), you can convert basically any format to .Mobi (Kindle's format), or ePub (which I think iBooks and maybe other ereaders use?), and then email if to your Kindle.

Using this, along with either archive (.org or .is) for public domain works, or LibGen (for just about anything else. Especially saved me buttloads on textbooks back in the day lol), you can get basically anything you want. Libgen can be a bit weird to use, but you basically just search for what you want, click on one of the mirrors (usually labelled on the right "1-6" or so), and then at the top, you should see "Get" — clicking that will start the download (sometimes takes a bit, other times you might need a different mirror. I may have made this sound difficult but it's very easy)