Have a look in "Attributes" panel to see if the object or objects to see if the Overprint Fill or Overprint Stroke have been ticked. Also have a look at this plugin http://www.astutegraphics.com/produc...asm/index.html it is a bit tasty in price but the designer plugin can be bought for $49. Please remember that this is basically a print command which has been given if you have received only the PDF from your client and not the .AI file.