Install PDF Creator (link to it on your downloads page) if you don't already have it or som other PDF 'virtual printer' installed.
Print the drawing - select PDF Creator, (or Acrobat, or PrimoPDF etc) as the printer (check that paper size is set correctly), press OK choose where to save the file.
[EDIT - once you have PDF creator installed (which Alibre Design expects) you also get an PDF export option in 2D, but it behaves slightly differently (changes the scale a bit and and adds header/footer) you might want to experiment with both.]