Presumably, you are finding that you cannot sign.
1. What software are you using, including FULL version number?
2. What steps do you follow to sign?
3. What happens exactly including any messages in full?
Thanks for the response. that's correct, I cannot sign the document.
The version using is Adobe Acrabt 9 Pro.
After open the PDF, simply click on sign. However All the option are grey out.
Where it indicates that signing is allowed, what it really means is: "If there are any existing digital signature fields present and they are unsigned, you will be able to digitally sign them, unless they are configured in a way that restricts how they can be signed."