The Kindle doesn't handle PDF natively, and if you use Amazon's conversion service, it's basically converting the PDF in the same way calibre does - with varied results. It sounds like your books are all text, and my guess is that they would convert quite well.
Download calibre, import a couple of books and try to convert then. Then you can use the viewer to see how the end result will look like on a reader.
Remember that if you've bought multiformat books at Fictionwise, you can download in another format.