How to modify password and security settings for pdf file in C#?

I'm an intern working on a program that uses the OpenXml SDK to modify some info in a word doc and then convert it to PDF using interop. Unfortunately I am unable to programmatically password-protect the PDF or change any of the security settings with interop. I'm looking for a way to do this that doesn't require me to release my source code or require my employer to pay for a software license. I've heard that I might need to write a plugin in C++ and then use unmanaged exports to call it in C#. Is this true? What are my options?