After you XRef in the PDF, select the PDF, then this tab should pop up (Note: I only copied the left side of the tab
There should be a fade slider on the tab.
Alternately, a numerical value shows up on the properties tab under the heading "Underlay Adjust"... There's Contrast, (1-100) Fade (1-100), Monochrome (Yes / No) and Adjust Colours for Background (Yes / No), you could enter a value in the Fade field instead of using the slider.
There is no way I know of that you would be able to use to modify the actual lineweights of a PDF. In a vector based PDF with layers included (ie a PDF made using the DWG to PDF converter), you can turn off layers but I have not found a way to actaully modify their properties.
Thanks, It seems fade is what I was looking for. When I fade the PDF it seems to print 'lighter' which is what I wanted. Basically I was drawing over the top of the pdf and wanted the pdf to be a lighter 'shade' as a background.