DXF and DWF are different - QCAD as far as I can tell only opens the former. As far as I'm aware, DXF is a fairly old file format designed for the exchange of CAD files between software packages (think it stands for Drawing eXchange Format). DWF is intended as a drawing-focussed competitor to PDF. It's promoted as being better for reviewing work - undertaking markup and measurements in a way relevant to drawings is supposed to be easier and more accurate than with PDFs. Bricsys are close to releasing a Linux version of BricsCAD - a program I use all the time at work (on a Windows machine). From my experience it's easily the best AutoCAD clone (and far less of a resource hog than current versions of AutoCAD, though predictably that does come at the expense of some of AutoCAD's newer features). BricsCAD has a range of DWF export options. Perhaps they'll be our best bet for bringing a DWF viewer to Linux, given it's evidently a format they're famiiar with.