A DWFx document is an AutoCAD DWF file formatted to work for Microsoftâ€™s XML Paper Specification (XPS). This is included with Vista and viewers are available for other operating systems. XPS is a direct competitor to Adobeâ€™s PDF technology and is better suited for graphics intensive documents.
You need the Microsoft XPS Viewer to open a DWFx document. This is already integrated into Vista, but it is also installed with the .NET 3.0 Framework or as a download for Windows XP. The viewer is built on the Windows Presentation Foundation and will work on Windows XP, 2003, Vista, and higher.
I would recommend the Autodesk Design Review application for working with DWFxâ€™s because it provides greater functionality than Microsoftâ€™s viewer. The application is a free download from Autodesk.