Plenty of free e-books in Mobipocket format show up fine on 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.
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.
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