Ubuntu – 2D CAD application for Ubuntu 14.04 —> output DWG/DXF files

For 3D use Blender. I think it is the only real alternative. BUT you will have to do a lot of work, because it is focused on animation. Blender is extremely wide, you will have to find what you need, what is not always easy for beginners. For example, there are plugin tools to improve exact 3D modelling, you might customize views etc.

For 2D use DraftSight (not free software). It is a clon of AutoCAD, which means that you don't have to get familiar with new commands, workflow etc. For individuals it is free, Information about your behavior will be send regularly to DraftSight. Go to their Homepage and get the actual .deb file. DraftSight's disadvantage is, that there is no 3D.

You can use QCAD, an open source AutoCAD replacement or the community fork LibreCAD made to work using the newer Qt4 framework. You can install LibreCAD from the Ubuntu Software Center. Using one of these two programs you import DWG files, edit the drawing and export it to PDF.

QCad offered a command line tool to convert DWG to PDF directly - I'm guessing it's also available in LibreCAD.

./dwg2pdf.sh drawing.dwg