I am trying to create a program which can add password to PDF file.
Can you guide me to to add password using SDK?
You would have to write a plugin for Acrobat Pro (not the free Reader) using C++. You cannot use VB or C# or write an app.
The C++ SDK contains examples of setting a password in the Snippets.
You can batch secure PDF files using the Encrypt Action in the Action Wizard.