Using Acrobat (Standard/Professional) version 9 can easily done the task. The requirements are:
- Having a digital certificate (I choosed VeriSign - US$19.95) from Windows Cert Store
- Extract the Root Cert from Windows Cert Store (for my case, is Verisign Class 1 Public Primary Certification Authority)
Sign the PDF with the purchased certificate and ask the PDF file viewer to import the "Verisign Class 1 Public Primary Certification Authority" root certificate into Acrobat Reader. I think asking people to import a VeriSign root cert is not too difficult. (At lease it is easier to ask people to import your own CA cert.)
At this point, you get cheap signing PDF solution. But the signature will show "The time is based on the local time on the signer's computer." To avoid this, I also use Digital Time Stamping service from DigiStamp.
Details of how to setup DigiStamp with Acrobat (Reader/Professional) can be found here.