Mixed Raster Content
You can reduce the size of a PDF by choosing the mixed raster content method. MRC compresses or removes the color background of a document so that only text and pictures are shown against a white background. The picture objects themselves (photos, diagrams, etc.) may also be slightly compressed.
As you can see in this screen grab when saving a PDF in Nuance Power PDF Advanced, you have the option to select, 'MRC PDF Document'.
If you click on settings, you can adjust the conversion rate. Minimal will generate the smallest possible file. Good will reduce image quality somewhat. Lossless preserves the original image quality. Using this method with 'Good' option I was able to reduce on 52 MB file to 4.5 MB this evening.
Here we can see the slight reduction in image quality between the compressed image on the top, and the original image on the bottom. Not much of a compromise.
Choosing the Minimal option would only reduce the file size a few KB more.