I found a way to do this but I bought a old version of Photoshop to do it. If you have photoshop, let us know and I will attempt to explan the steps, Im a beginner learning to decrease book file size create a PDF library from my books, mostly from mixing all the good ideas other posters have made.... I have learned alot over the past 4-5 months, eventhough I only half way know what I am doing!
You can generally enhance the contrast of a scanned document in Adobe Acrobat using the: PREFERENCES/ACCESSIBILITY / REPLACE DOCUMENT COLORS drop down menu.
Thanks!!!! Worked great to darken the text on my faded-looking scan!
I'm leaving this comment here for any future users needing help solving this problem as I did. Fortunately I found that some of the most common image editing software applications can open PDF's.
If you open your PDF in GIMP you can then adjust contrast/brightness, curves, levels, you name it.