Files with the extension .mxd are ArcMap document files. MXD stands for Map Exchange Document. Only products related to the ArcGIS suite produced by Esri can read and open MXD files. ArcMap document files contain map, specifics about the GIS data used (including pointers to the file location for each dataset), display information (symbology and labeling) and other elements used in ArcMap. ArcMap document files don’t physically store GIS data but contain pointers to the location of the GIS data. GIS data that is not locatable when opening an ArcMap document file will be noted by an exclamation point (!) next to the data symbology in the legend section of the ArcMap table of contents.
Exporting MXD to other file formats
Natively, ArcMap layouts can be exported to a range of graphic file format including PDF files. To export a MXD file to QGIS format, Allan Maungu developed an ArcToolBox called MXD2QGS that will export layers from ArcMap to QGIS format.
Reduce MXD file sizes
To reduce the size of an ArcMap document files file, run the ArcGIS Document Defragmenter utility. This utility defragments the storage with a MXD file to reduce its file size. DocDefragmenter.exe can be found in the Tools folder of the ArcGIS Developer Kit.
CompactMXD.zip can also reduce ArcMap document files file sizes up to 60%. To use, open up ArcMap (without selecting a file), run CompactMXD.exe, navigate to the MXD file(s) you want to compress, select the MXD files and click compress. The new compacted files will have .compact.mxd as the new extension.
Fixing MXD files
Broken ArcMap document files can be fixed using the MXD doctor utility. The utility is a standalone (.exe) files located in the Tools folder of the ArcGIS Developer Kit. The utility first analysis the state of the broken file. The utility analysis the various components that make up an ArcMap document file: Maps – Data Frames, Page Layout, VBA Project, Style Gallery, and Thumbnail Preview. The results of the analysis then indicate the recoverable status of each of those components. Recoverable entities can then be copied into a new or existing .mxd file.