One thing I can recommend to reduce the file size of the scanned pdf document is to open the original pdf which is of large size in adobe acrobat reader or Adobe Acrobat professional and click on save as option to save another copy of the same file and check the size, you will sure have the file size reduced. this helped me most of the time in reducing the pdf file sizes, there are huge number of softwares available, try Google.
There is good article on how to Reduce PDF File Size at Adobe. For your more information there has been adobe acrobat 9 released recently, you can download the trial versions to check the exciting features in Adobe 9 Pro. Discuss using the Comments section at the bottom.