Decent PDF compression products will segment scanned images into components and compress each according to content. This results in much smaller scanned pages which look identical to the original.
- Nuance Omnipage which uses MRC compression.
- Solid PDF Tools which uses CGM compression (my company's product).
Typically these kinds products will break the pages into parts that make sense from a compression point of view:
- photo-like images are downsampled (say from 300dpi to 150dpi) and then compressed with lossy compression (Jpeg or Jpeg2000)
- black text is compressed using lossless compression (CCITT Fax for example)
- "flat" color (like graphs, etc) is converted to palettized images.