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MOBI is the format used by the MobiPocket Reader. It may have a .mobi extension or it may have a .prc extension. The extension can be changed by the user to either of the accepted forms.
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MOBI format was originally an extension of the PalmDOC format by adding certain HTML like tags to the data. Many MOBI formatted documents still use this form. However there is also a high compression version of this file format that compresses data to a larger degree in a proprietary manner. There are some third party programs that can read the eBooks in the original MOBI format but there are only a few third party program that can read the eBooks in the new compressed form.
Não há fontes citadas, tanto quanto eu posso dizer, mas o take-away é que .MOBI
é uma extensão ou prc
e alguns recursos não são suportados pela maioria dos leitores.