IMHO Ask the file creator to "save as autocad" to knock down their custom AEC content to plain AutOCAD object types (if that is what those objects are created with). the free viewer is not the best tool to do so.
If that's not an option, set the DWGCONVERT tool inside DTV to export to 2000 ro even R14 format and remember to uncheck the "maintain visual fidelity" box when you go into the setup option.