Convert a PDF to Grayscale

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.