string ownerKey = Convert.ToBase64String(CryptoHelper.RandomKey());
    using (PdfDocument pdf = PdfReader.Open(tempPdfPath, PdfDocumentOpenMode.Modify))
    {
        pdf.SecuritySettings.OwnerPassword = ownerKey;
        pdf.SecuritySettings.UserPassword = string.Empty;
        pdf.SecuritySettings.PermitPrint = true;
        pdf.SecuritySettings.PermitModifyDocument = false;
        pdf.SecuritySettings.PermitExtractContent = false;
        pdf.Save(tempPdfPath);
    }


    internal static class CryptoHelper
    {
        // Return 16 random bytes (128-bit) every call.
        internal static byte[] RandomKey()
        {
            byte[] key = new byte[16];
            RandomNumberGenerator rng = RandomNumberGenerator.Create();
            rng.GetBytes(key);
            return key;
        }
    }