Hi pedrosalpr,
Beginning with Reader version 11.0.7 (which just shipped a month ago) we started allowing the addition of a digital signature without the document creator (in this case that's you) having to Reader Enable the file. However, since you can't count on all of the people that are going to sign the file having upgraded their version of the free Adobe Reader you should go ahead and Reader enable the file using Acrobat before you try to sign it in Reader. From the File menu select Save As Other > Reader Extended PDF > Enable More Tools and save the file. The new file can be signed using a version of Reader older than version 11.0.7.
Steve