.MXD File Extension
What is an MXD file?
Map file format used by ESRI mapping software, such as ArcMap; saved in a compound format that includes the map description, layout, and embedded objects saved in the map; organized into units called storages and streams.
NOTE: MXD map files may grow in size after being saved multiple times because they can become fragmented. Saving a fragmented map document as a new document may reduce its file size.