The Kindle only allows the reading of Amazon DRM-protected content.
Typically ebook readers are larger than PMP's with larger screens. If you're going to read for a few hours, then the screen of a typical PMP becomes pretty eye-straining (due to size, contrast, and brightness).
The Kindle DRM wasn't hacked to REMOVE the DRM from Kindle purchases, but added to other books so that they too could be read on the Kindle.
Granted the PDF converter seems to have some trouble with graphics PDF's, but for books the converter works very well.
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