The PDF has to be DRM (and password) free.
Note that it is now possible to strip Adobe PDF DRM, see
Adobe ADEPT DRM for PDF circumvented. If you have a PDF with a password, it is also possible to produce a password-free version.
I would not personally send any file to Amazon that started out with DRM. Amazon probably does not check the files, but why take the risk when there are alternatives (e.g. Calibre or Windows MobiPocket Reader/Creator).