To able to digitally sign a document using Adobe Reader, it is mandatory that you have your digital certificate properly installed on your computer.
You will find in this webpage the steps to follow, using Adobe Reader DC and bellow using Adobe Reader XI
Open the document you want to sign in Adobe Reader DC.
Select the option "Tools” (1).
Select the option "Certificates” (2).
Click on the button “Digitally Sign”.
Select with the mouse where you want the signature to be displayed.
Select the digital certificate that you want to use (1) and click on "Sign" (2).
Open the document you want to sign in Adobe Reader XI.
Select the option "Sign” (1).
Press the option "Place Signature” (1).
Click on the button "Drag new signature rectangle...” and select where you want the signature to be displayed.