如何保护用iTextSharp签名的pdf文件？

Question

我正在使用iTextSharp签署pdf文档。它工作正常，但我想保护该文件，我希望该pdf签名不能再次打印，复制或签名，该怎么办？

我尝试过

PdfEncryptor.Encrypt(
    reader,
    new FileStream(this.outputPDF, FileMode.Create, FileAccess.Write),
    true,
    null,
    null,
    PdfWriter.AllowFillIn | PdfWriter.AllowScreenReaders);

但是不起作用，它说

  

iTextSharp.text.DocumentException：'原始文档已被重用。从文件中再次读取。'

public void Sign(string SigReason, string SigContact, string SigLocation, bool visible)
        {
            PdfReader reader = new PdfReader(this.inputPDF);
/*ERROR-->*/PdfEncryptor.Encrypt(reader, new FileStream(this.outputPDF, FileMode.Create, FileAccess.Write), true, null, null, PdfWriter.AllowFillIn | PdfWriter.AllowScreenReaders);
            PdfStamper st = PdfStamper.CreateSignature(reader, new FileStream(this.outputPDF, FileMode.Create, FileAccess.Write), '\0', null, true);
            st.MoreInfo = this.metadata.getMetaData();
            st.XmpMetadata = this.metadata.getStreamedMetaData();
            PdfSignatureAppearance sap = st.SignatureAppearance;

            sap.SetCrypto(this.myCert.Akp, this.myCert.Chain, null, PdfSignatureAppearance.WINCER_SIGNED);
            sap.Reason = SigReason;
            sap.Contact = SigContact;
            sap.Location = SigLocation;

            if (visible)
                sap.SetVisibleSignature(new iTextSharp.text.Rectangle(100, 100, 250, 150), 1, null);

            st.Close();

        }