You can also use the Amazon email service to convert PDFs into .mobi files. Just email the file as an attachment to your Kindle's email address (e.g.
yourkindlename@kindle.com), putting CONVERT in the subject line, and it will show up on your Kindle, converted to .mobi format. How well it converts depends on how fancily it was formatted. Fancy formatting doesn't translate well. Neither do pdfs that are just images of pages. Those don't work at all. Those require OCR software.