Not sure if DXF format supports bitmaps, but DWG 2007 file format or maybe even DWG 2004 file format sure do support bitmaps in drawings.
In default shaded views from Pro/E drawings always export as bitmaps, that's why you cannot display them in regular vector based CAD such as ACAD or Draftsight using an older format.
I didn't test this, but I guess setting the config.pro option force_wireframe_in_drawings to yes might take care of your trouble with existing drawings containing shaded views. So your customer, supplier or whoever can see the views at lest in wireframe.
I guess if the view is specifically set to shaded then it will just show as shaded anyway, according to that config.pro option description, but it might be worth trying out.