A-PDF To Black/White provides a quick way to convert the images to monochrome or grayscale in Acrobat PDF documents directly. It detects color images and make them to bw or gray automatically, then reduces your PDF files size. A-PDF to Black/White"s features include processing a batch of PDF files, even working with password protected files; filtering small images which may be a logo or else; and dealing out some page only to speed the processing. A-PDF to Black/White does NOT require Adobe Acrobat, and produces documents compatible with Adobe Acrobat Reader Version 5 and above.
Source: http://www.a-pdf.com/bw