"ACAD has different layers that make it handy to have drawings that you can select only those parts you want to see.
Also, if the dimensions are missing, it wouldn't be difficult to add them in ACAD if the drawing file were opened in ACAD rather than the viewing-only program"
Very true, Chim...but then you're looking at spending money. AutoCAD goes for a few thousand bucks now.
Of course, you can get AutoCAD Lite for less then a thou....or try eBay for an older version. There is also IntelliCAD which open .dwg and .dxf files.