Just tried this recently - wish I'd known sooner.
If a scan is produced in pdf format, you can drastically reduce the file size by:

1. Open in Acrobat 8 (don't know about earlier versions)

2. In the "Document" menu, select "Optimize scanned pdf." That's it. (A dialog box opens, but the default settings have been OK.)

Apparently a scanner picks up lots of "information" (dust, print artifacts, ?) that is not significant to the image. Using the "optimize" function has reduced 400 - 500 kb files to about 100 kb or so, on average. Just scanned two 8x11 sides of a doc in grayscale with a result of about 324kb when both pages were combined to one file. Running "optimize" reduced it to 60 kb. AND straightened a very slightly crooked image at the same time.

I just used the default settings in the "optimize" dialog. For more info on what they mean, look up "Optimization Options dialog box" in Acrobat Help.