What is an MXD file?
An MXD file is a Map Document file used by ESRI ArcMap software. It is a proprietary file format that stores different type of information such as the layout and design of maps, data used to generate maps and the symbology applied to the data. MXD file is, thus, a compound file format that contains several different type of map elements in a single file. MXD files is used by the ArcGIS Map Package (.mpk) file.
MXD File Format
MXD files are saved in ArcGIS proprietary file format and its internal details are not available for developer’s reference. The embedded elements of an MXD file are organized into units, known as storages and streams.