There is only one file type that uses the .MJD extension:
Adobe Acrobat MIME Encoded Job Definition File
Package file created by Adobe Acrobat, a PDF editing and publishing program; contains a MIME-encoded archive of PDF documents and a .JDF job ticket file; used for transferring document publishing workflow packages over FTP and email.
Since there often are multiple groups involved in the printing process, MJD files can be used as a platform-independent format that is safe for delivery over email and the Internet. When receiving a MJD file, you can open it in Acrobat to extract and view the package's contents.
Updated: August 9, 2010