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. In either case it may be DRM protected or non-DRM. The .prc extension is used because the PalmOS doesn't support any file extensions except .prc or .pdb. Note that Mobipocket prohibits their DRM format to be used on dedicated eBook readers that support other DRM formats.
You can convert MOBI to PDF by our online program, PDFConvert desktop software or command line program.