There are many suggestions but they usually apply to documents. They do not work on scanned papers. Is there a way to decrease the contrast of a pdf image altogether?
I would suggest converting the file to JPG to be able to edit the brightness and contrast. You can use free online sites such as smallpdf.com to convert the file from PDF to JPG, then adjust the contrast in a picture editing program. If you don't have one installed already, you can find plenty online.