A technical writer at work sent me a document for review, and told me to use Adobe's commenting tools to annotate it. I open it up in Reader and -- surprise -- there aren't any commenting tools!
I expected this to happen, because it wasn't the first time this had happened to me. Adobe Reader can only annotate PDF documents when the author has enabled specific usage rights. Apparently most technical writers either don't understand this, or they all have the full Adobe Acrobat and simply have never encountered the problem.
The quick solution: grab PDF-XChange Viewer (which allows mere mortals to annotate any PDF document), and leave the Adobe mess for someone else.
p.s. the small "portable" version includes all the commenting tools (second Download button in the link above, as of this writing) ... not sure why the full version is so much bigger.