This is an update to my post Howto: Add a Digital Signature to a PDF File, but this time I found free software.
Select the PDF file to sign and select an output file (if you don’t want to overwrite your original):
Push Sign It:
And then you can check the signature with Adobe Reader:
If you get the following error, make sure you change your PDF version from %PDF-1.1 to %PDF-1.4:
Choosen configuration requires PDF version update, but it’s not possible in the “append” signature mode.