The easiest way to convert a full-color PDF into grayscale is to use Acrobat 8 and above.
Choose Advanced> Preflight… then under Preflight profiles choose PDF fixups. One of the fixups is named Convert to grayscale.
While you’re there, check out the other options, such as Convert to CMYK, Flatten various things, Make spot color names consistent, and convert to sRGB.
Jay Nelson is the editorial director of PlanetQuark.com, and the editor and publisher of Design Tools Monthly. He’s also the author of the QuarkXPress 8 and QuarkXPress 7 training titles at Lynda.com, as well as the training videos Quark includes in the box with QuarkXPress 7 . In addition, Jay writes regularly for Macworld and Photoshop User magazines and speaks at industry events.