I reduced a 14MB PDF (high res scan, only 20 pages) to 800kb just by trimming borders, scaling all pages to 1024x768 (on the kindle that would be 800x600) and reducing color depth to 16 gray levels (while making the background perfect white). Of course by doing so you lose any and all zoom since there just isn't any more detail to the picture then. But without zoom it doesn't look any different either way on the reader.
Try not to use OCR at all unless you want to go away from PDF. Of course the point is moot if you want/need search, reflow. toc could probably be done manually without ocr...
2000+ page pdf scan will probably not be possible. You'll have to split the book.