Apparently you are talking about scans. You can do this with your scanner settings before you get it into Acrobat. You can also edit the PDF with a graphics package (PhotoShop) to increase the contrast or whatever.
This presumes you have access to the original paper so you can rescan it. If you don't, are there any levels/contrast adjustments avaible in AA9 though? (I haven't seen any.)
There is nothing other than the tools available under Document>Optimize scanned PDF.