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How can I load an ebook in ePub Un-encrypted (DRM free) format on to my PRS-300?
Get a non-DRM version.....
If it's not a pub-dom work, or not issued in a non-locked form, you can't load anything on a non-compatible device.
If purchased legally, whether it's fair/reasonable that you can't do what you want, ( i.e. load it onto the device of your choice ) with something you've paid for,(i.e. an intellectual property rather than a physical ) is another can of little wriggly things altogether ...