The Kindle essentially only reads MOBI ebooks.
If your PDFs are DRM-free (e.g. you can open them in Adobe Reader) then you can convert them to MOBI. One way is to e-mail them to Amazon for conversion, see
Transferring Documents and Files to Kindle. Mail to @free.amazon.com to get back the MOBI via email (rather than transferred via whispernet for $0.15 per MB). There are also options on your desktop computer. Windows MobiPocket Reader will import PDFs and convert them to MOBI (output typically in My Documents\My eBooks). Calibre (has its own forum) can do the same.
If the PDFs are secure ebooks (only readable using Adobe Digital Editions), then you will have to first strip the DRM - see
Adobe ADEPT DRM for PDF circumvented. It discusses the legal situation with removing DRM as well as how to do it.