Adobe ACROBAT READER 7.0 Manual page 70

Previewing how colors overprint
Overprint Preview mode lets you see (on-screen) the effects of ink aliasing in the printed
output. Spot colors aliased to other spot colors or to process colors are reflected directly in
the open document.
To preview how colors will overprint and blend:
1. With the PDF file open, choose Edit > Preferences (Windows) or Adobe Reader >
Preferences (Mac OS) and select Page Display on the left.
2. Select Overprint Preview.