You sometimes want your PDF to be in GreyScale or black and white as it were. It can make it look more uniform, or if you had a flaky scanned document and the color looks off.
Anyway, it is quite easy to convert a document to Black and White in Acrobat Pro 9.
1) Open document. Duh.
2) Click Window –> Preproduction –> PreFlight
3) CLick the down arrow under PDF Fixups.
4) CLick the Convert to grayscal row/option.
5) CLick Analyze and Fix(bottom right hand corner).
6) Pick new filename. Voila! You are done.