I had posted last week asking for help but the answer I was given did not fully answer my question. I posted a follow up but no one has responded to that.
Hi, sorry, this was not my question.
Document document = new Document("c:/pdftest/SimpelPDFfile.pdf");
document.Encrypt("user", "owner", 0, CryptoAlgorithm.RC4x128);
//save updated PDF