Is the only way to make text in a PDF darker to use Document-->OCR Text Recognition-->Recognize Text Using OCR and then use the ClearScan PDF Output Style?
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.