There aren't any settings that I know of off of the top of my head. When I get back into the office next week I can play with the novaPDF library a bit to see if there is any tweaking that can be done.
Ok, after a bit of digging this is what I've come up with. The ReportsPLUS uses Crystal reports built-in pdf generator which we don't really have any control over (that I can tell). However, what you can do instead is go to "File > Print Setup" and choose the NovaPDF printer as the printer to use, then click on the properties button as shown here:
In the properties screen, go to the lower right corner and change the Graphic resolution to 1200dpi (lower may be ok, I suggest you play with the setting):
Once this is done, you can simply ok out of these screens and "Print" the report, it will ask you for a file name to save it too. The images should be much crisper than before.