Using maximum zip compression in WinRAR, I did a test for you (you could have done this yourself!)
26/07/2008 19:56 1,968,333 mg_barcelona.pdf
07/09/2009 10:08 1,937,667 mg_barcelona.zip
So not much. PDF files are like mp3, wmv, mpeg, and jpg files; already compressed, so that no compression algorithm can find any big savings to make. I even downloaded a "free" "pdf compression" program and the output file was 100% of the size of the original.
So the answer is, don't bother; PDF format is already optimized for web distribution, and using zip will only introduce an un-needed step that your customers will have to take, making the process less easy, for no good reason.