Plenty of free e-books in Mobipocket format show up fine on Kindle.
We've known for some time already that Amazon's AZW files are actually Mobi files, but Amazon didn't share Kindle's Mobi PID which would allow one to buy encrypted Mobi books for Kindle.
Use Mobipocket software to convert PDFs to that format so you can read them on Kindle.
Non DRM Mobipocket format is already supported on the Kindle - just drag and drop.
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