If you have Distiller, it is quite simple.
Save the PDF as a .ps file in Acrobat Reader
Open Distiller, and under Settings->Job options->Compression
, specify your downsize resolution
If you want a PDF for viewing on monitor
only, downsize all graphics to 72 dpi
, or 90 at most.
That will probably reduce your file quite a bit
After setting the options, open the .ps file in Distiller, and it will spit out a much smaller PDF.
Note, that it might get the same name as the original, so make a copy of the original, so that you don't overwrite it. The process is irreversible :-)