Question: I want to optimize my Adobe PDFs. What is the best way to optimize Adobe PDF documents?

Answer: One of the most important factors for optimizing Adobe PDFs is compression. Just because a file has lots & lots of pages, does not mean that the file size must necessarily also be large. Compare, for example, a 1,000 page electronic file with a 1,000 page scanned color TIFF file. The scanned file can easily be 100x larger than the electronic file (e.g., 1 GB vs. 10 MB). So compression can be a key factor in making sure your documents are “optimized.” Compression is particularly a factor when dealing with scanned image documents. Compression can yield reductions of up to 10x for black and white image documents and up to 100x for color image documents.

See, for example, http://www.cvisiontech.com/pdf_compressor_31.html