Converting a color PDF to Grayscale is easier than you think. This process will work on CMYK, RGB, and Spot colors found throughout your entire document. You will need Adobe Acrobat Pro in order to do this. (I think version 8 or higher).
1: Open your document
2: Find the Advanced menu – Select Print Production – then Preflight
3: Select PDF Fixups – then Convert to Grayscale
4: Click Analyze and Fix then re-save the document.
Your done! Notice that the entire PDF is now grayscale (The document size will probably be a little less too).