There are converters available from Amazon and/or Mobi for pdf and doc files.
Hack let you convert encrypted Mobipocket files that were intended for another reader so that that work on Kindle.
Plenty of free e-books in Mobipocket format show up fine on Kindle.
Granted the PDF converter seems to have some trouble with graphics PDF's, but for books the converter works very well.
A revolutionary portable reader that wirelessly downloads books, newspapers, magazines and blogs to a crisp, high-resolution electronic paper display that looks and reads like real paper, even in bright sunlight.