The Kindle only allows the reading of Amazon DRM-protected content.
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.
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.
Use Mobipocket software to convert PDFs to that format so you can read them on Kindle.
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