You can also try Nuance Paperport (I got this "free") with my Brother MFC scanner/printer.
Essentially if you have a dark grey font on a light grey background (a low contrast scan), then you tell it to "stretch" the dark grey to black and the light grey to white. This is done by setting the black/white points as follows:
- Open Paper port, navigate to the PDF
- Right click the PDF within PaperPort
- "SET Tools"
- Auto-enhance (or Apply current white/black points)
Now you can manually set the while/black points for finer control
- Open Paper port, navigate to the PDF
- Right click the PDF within PaperPort
- Open with ImageView
- In the top ribbon, pick "White point" and now click some area of the scanned page you think should be white (eg: grey background of a low contrast scan).
- In the top ribbon, pick "Black point" and now click some area of the scanned page you think should be black (eg: The grey text letters of a low contrast scan)
I used this to create a legible black and white PDF from a scan that was originally black text on dark blue paper (that scans as a very low contrast)