This is perhaps a silly question but can ArcGIS Explorer open a .mxd file? I can't seem to find a definite answer online and I can't download it until I know the answer. Thanks.
mxd map document cannot be opened in ArcGIS Explorer.
You can add layer packages and data such as some but not all ArcGIS online basemaps, OGIS web mapping service (WMS) data, a KML/KMZ files, geodatabases, shapefiles, GPX, and rasters JPEG 2000, GeoTIFF, MrSID rasters. As for layer packages; these have to be first created and uploaded into ArcGIS online via ArcMap before they can be opened in ArcGIS Explorer. (1200 or more recent)
Unfortunately, you cannot open .mxd files in ArcGIS Explorer
You can open these item types in ArcGIS Explorer Desktop:
Please see this page: http://help.arcgis.com/en/arcgisonline/help/index.html#//010q0000000v000000