On certain documents some of the text is light to begin with. After the scanning
You cannot read some of the text. Is there some software that will darken the light
Text so you can read it.

Almost any image viewing program should have a way to do this. Look for a Contrast or Gamma adjustment. You might also try Sharpen, if available.

A wonderful free program that I use is Irfanview (http://www.irfanview.com).

Since your original material isn't that great, don't expect excellent results.

Hope this helps,
WallyH

Try a contour filter

If you cannot read the text before scanning there is no magic to undo the physical damage