It’s an old thread, but I just stumbled on the same problem and didn’t want to install anything additional, so I suggest this: (building on what nmat says and the comment of slhck)
If you want to reduce margins, then, in the PDF reader you could print-to-pdf with scaling to 110% and then print the new PDF the way you want on paper (single or both sided or multiple per page). With this solution you can only increase decrease all the margins (top, bottom, right and left) proportionally; i.e., you can not change only one margin.
Edit/Update: I came across Briss, a free (GNU GPL) and quite compact application (Java) you can download from SourceForge (briss-0.9 currently). It can remove the margins of PDFs and save the result. I’ve been using it for few months already.