Adjust contrast and brightness of a PDF1/05/2012
In case you want to adjust brightness and contrast of a PDF file on the Mac (or Linux) you can use ImageMagick. Here is what I used:
convert -brightness-contrast 10,50 -compress lzw \ -density 500 -colorspace Gray input.pdf output.pdf
the -brightness-contrast 10,50 controls the brightness and contrast (they can both take values from -100 to +100), Density controls the resolution and -colorspace controls the color output. In this case, its Gray. lzw is a lossless compression method.