What does it show when you select: File > Properties > Security
It said Signing: Not Allowed.
The Properties box says to Save A Copy, which is not an option under my file menu. I used Save As, but the restrictions were still in place.
It's the version of the W-9 Rev. Oct. 2007. Could that be the problem? With past versions of Acrobat, I never had a problem applying a digital signature.
You should have a "File > Save a Copy" menu item in Acrobat 11. This will save the form as a non-Reader-enabled version that you should be able to sign with Acrobat, but I can't say for sure without looking at the actual file. Do you have a link to it online?
Ooops — I was looking in the wrong place. The Save A Copy option isn't grouped with the other Save / Save As / Save As Other options. It's further down. Will give it a try.