The following question and answer on StackOverflow show how to generate a PDF that cannot be opened without the appropriate password.
Password protected PDF using
Yes, there are two passwords that you can pass to PdfEncryptor.Encrypt(), userPassword and ownerPassword. Just pass null to the userPassword and people will be able to open it without specify a password.
string WorkingFolder = Environment.GetFolderPath(Environment.SpecialFolder.Desktop);
string InputFile = Path.Combine(WorkingFolder, "Test.pdf");
string OutputFile = Path.Combine(WorkingFolder, "Test_enc.pdf");
using (Stream input = new FileStream(InputFile, FileMode.Open, FileAccess.Read, FileShare.Read))
{
using (Stream output = new FileStream(OutputFile, FileMode.Create, FileAccess.Write, FileShare.None))
{
PdfReader reader = new PdfReader(input);
PdfEncryptor.Encrypt(reader, output, true, null, "secret", PdfWriter.ALLOW_SCREENREADERS);
}
}