How to send pdf files through email and the file should be password protected.

I have a requirement in this the user would load pdf files from a front end interface and those files will finally go and reside in fnd_lobs and fnd_documents_vl. I am using UTL_SMTP to send the pdf files and it is working fine.

I want to send the pdf file and it should be password protected. How can this be achieved. Kindly share the details and your views. Can this is achieved through PL/SQL and/or what are the different ways of tackling this one.

--Tanay