如何在C#上将pdf数字签名到单独的文件中？

using System;
using System.IO;
using iTextSharp.text;
using iTextSharp.text.pdf;
using Org.BouncyCastle.Crypto;
using Org.BouncyCastle.Pkcs;
using Org.BouncyCastle.X509;

public class PdfDigitalSignature
{
    public static void SignPdf(string sourceFilePath, string destinationFilePath, string certificateFilePath, string certificatePassword)
    {
        // 加载PDF文件
        PdfReader reader = new PdfReader(sourceFilePath);
        FileStream os = new FileStream(destinationFilePath, FileMode.Create);
        PdfStamper stamper = PdfStamper.CreateSignature(reader, os, '\0');

        // 创建数字签名
        PdfSignatureAppearance appearance = stamper.SignatureAppearance;
        appearance.Reason = "Digitally signed by [Your Name]";
        appearance.Location = "Location";
        appearance.SetVisibleSignature(new Rectangle(36, 748, 144, 780), 1, "Signature");

        // 加载证书
        Pkcs12Store store = new Pkcs12Store(new FileStream(certificateFilePath, FileMode.Open), certificatePassword.ToCharArray());
        string alias = null;
        foreach (string al in store.Aliases)
        {
            if (store.IsKeyEntry(al) && store.GetKey(al).Key.IsPrivate)
            {
                alias = al;
                break;
            }
        }
        AsymmetricKeyEntry keyEntry = store.GetKey(alias);
        X509CertificateEntry[] chain = store.GetCertificateChain(alias);
        IExternalSignature externalSignature = new PrivateKeySignature(keyEntry.Key, chain[0].Certificate, "SHA-256");

        // 进行数字签名
        MakeSignature.SignDetached(appearance, externalSignature, chain, null, null, null, 0, CryptoStandard.CMS);

        // 关闭PDF文件
        stamper.Close();
        reader.Close();
    }
}