DRM is not to prevent other files from being readable on the Kindle.
There are converters available from Amazon and/or Mobi for pdf and doc files.
Non-DRM Mobi books, like free e-books found online in Mobi format, or files you convert to Mobi files, are already supported on Kindle.
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).
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