MPJ is file extension used by help content creator tool called Adobe RoboHelp. MPJ is the main file generated by RoboHelp and stores information about source files for given help content.

MPJ project file holds various data such as the actual content (topics and their location, images, indices, etc.), project properties (for example, a title and language in which it was written), and navigation data. MPJ files were replaced by XPJ files that use XML format for storing data, but MPJ are still supported by newer versions of RoboHelp.