I am new to the phenomenon of scanned documents using a color scanner being very large in size. An 8 page document that was about 105K when originally delivered to me as a PDF file ended up being about 12.5M after I had printed it out, signed my signature and scanned it to a PDF file again.
Looking around the web, it seemed like there were many suggestions on reducing the size of a scanned document. There are software that will compress the size of your scanned PDF file, but nothing free. Ultimately I played around with the settings of my scanner (which is a Canon MP620 Pixma printer/scanner/copier combo) and found that the following settings reduced the size of the scanned image drastically.
The single change that had the biggest impact was to change the scan settings so that the document was scanned as a Black & White image instead of a Color image. Another setting that reduced the size further was to use 150dpi when scanning instead of 300dpi which was set as the default. Finally a big reduction results when you choose high compression when converting to a PDF file. Finally I went with just the Black & White scan setting only (since changing the other settings also reduced the quality of the resulting PDF file) and I was able to get a PDF of about 2M instead of the 12.5M that I was originally getting. So a decent amount of reduction in size.