Password protected PDF using C#

I am creating a pdf document using C# code in my process

Are you using some library to create this document? The pdf specification (8.6MB) is quite big and all tasks involving pdf manipulation could be difficult without using a third party library. Password protecting and encrypting your pdf files with the free and open source itextsharp library is quite easy:

using (Stream input = new FileStream("test.pdf", FileMode.Open, FileAccess.Read, FileShare.Read))
using (Stream output = new FileStream("test_encrypted.pdf", FileMode.Create, FileAccess.Write, FileShare.None))
{
    PdfReader reader = new PdfReader(input);
    PdfEncryptor.Encrypt(reader, output, true, "secret", "secret", PdfWriter.ALLOW_PRINTING);
}