Have you tried the two step PDF creation process? First, print to file, then launch Distiller, open the postscript file, then distill. This works better than the direct 'Export to PDF' choice in PM, as you have more control over the Distiller settings.
You need a postscript printer driver for this to work - you can download a PS printer driver from the Adobe site. You don't have to actually have a real postscript printer attached to your computer, since you are 'printing to file', and for which you need a 'virtual' PS printer. When your PM file is ready to PDF, go to 'Print', select the Adobe PS printer, check the box labelled 'print to file', make sure any TIF images are set to 'normal', (not 'optimised') and OK. This will make a postscript file with the extension .ps
Then launch Distiller, open the .ps file, choose your compression settings (dependent on intended final output**) and OK. You will be asked where to save and what to name both the postscript and PDF files.
** in V5, if final output is to a desktop printer, choose either the 'Print' or 'eBook' settings. If to be sent to a commercial printer for output, choose the 'Press' setting. If for viewing on screen only, chooose the 'Screen' setting.