My guess would be that it has to do with the PDF plotter and how AC renders "transparency"... Using a different viewer might help, but I usually batch plot to CUTE in these cases as it seems to convert ACs % mesh to proper shading. You would think the internal PDF plotter would translate it properly rather than rendering out the % mesh...
ps. Please add a signature to your profile. See link in mine.
pss. You can press "start + shift + s" to take a screen shot of a selected portion of your screen.