DRM refers to the fact that you can't view Amazon Kindle books on other devices (including your computer), or on other Kindles - i.e. no sharing, the purpose of DRM.
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.
The Kindle only allows the reading of Amazon DRM-protected content.
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