If you can, follow Lori's advice. If that is not an option, the you can get the best size reductions when you use the function to optimize a scanned PDF: Select Tools>Document Processing>Optimize scanned PDF. You can then use the size slider to adjust how much compression you get, but keep in mind that the smaller the size is, the more information gets removed, so you need to find a good compromise between size and quality.
Karl Heinz Kremer
PDF Acrobatics Without a Net
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