[mathfailure]

I once used it on a pdf file with a scanned text form trying to make it more contrast, as the scan was hardly seen. What I needed was basically just making dark stuff darker (up to the point of making it black) and maybe a bit thicker to make it more visible.

The tool failed to help me with such a seemingly menial task, the improvement was very small. I even tried to repeat the step multiple times, but after like 2nd use there were no visual differences anymore (but the file's size kept actually changing).

[jacob019]

That's not really a PDF thing. In that case the PDF is a thin wrapper around raster images. Extract the images, increase the contrast with gimp or imagemagick, and make a new PDF. You could script this if you had a lot of them.