Overprint:

[Albacore]

Yes you have got to give respect to such an old man!
If you're opening a PDF file in Adobe Reader and this is the way that I would go, and you want to see the file correctly on your screen, you need to turn on a preview feature called "Overprint Preview". Both Illustrator and InDesign have this feature (found in the View menu), but they've managed to do a good job hiding this feature in Acrobat: open Preferences and navigate to the Page Display panel. Then check the button marked Overprint Preview. The file will now appear correctly on screen and you will see the areas that will cause you problem.

If you're working on a file with overprints in Illustrator and need to simulate those overprints when printing from Illustrator, you can do so by choosing File > Print, navigating to the Advanced panel of the Print dialog, and choosing the Simulate option in the Overprints popup menu. This is the area where you can discard the Overprint option so you will need to use the print function to Adobe Acro, see screen grab.

Hope this helps and I would have a look at the help files in Illy it will give you a better understanding of what you are doing in this area as I agree with Gary here and would only think of doing this if I was using white or a large transparent area. With transparencies I would use the flatner. Come back if you need more help and also use the net for help as there are loads of info out there on this subject.