I want to sign a document in Acrobat, but the option is grayed out.

How was the PDF created?

Has security been applied to the PDF?

If you are using Adobe Acrobat Pro X, you have to log in first, in order to be able to sign a PDF document. Go to "Tools", "Sign & Certify", "More Sign & Certify", click now on "Digital ID files", click on the digital ID file you want, and if you see there "Logged out", click on "Login" at the top, and log in. Now, you will be able to sign your PDF document.
Dara Koper, Salzburg