This article explains the different file formats that use the ARD file extension. It also explains how to open each type.
A file with the ARD file extension may be an ArtiosCAD workspace file that contains a drawing or a 3D design. They are used with Esko's ArtiosCAD program.
However, your specific ARD file might instead be an Alphacam Router Drawing. We don't have any information on this format, but given the nature of the Alphacam Router software, it's most likely some sort of drawing file used to explain how a CNC router should cut something.
If the file is in neither format, it can be used with IBM's Content Manager OnDemand software. We're not sure if they're related at all, but ARD is also an abbreviation for asynchronous request dispatcher, a setting used by some IBM programs.