public partial class Default : System.Web.UI.Page
    {
        private const string UnsngnedPdf = @"C:\Users\vrshaikhislamov\Documents\esign\request.pdf";
        private const string TempPdf = @"C:\Users\vrshaikhislamov\Documents\esign\request_temp.pdf";
        private const string SignedPdf = @"C:\Users\vrshaikhislamov\Documents\esign\request_signed.pdf";
        protected void Page_Load(object sender, EventArgs e)
        {
            if (!string.IsNullOrEmpty(eSign.InnerHtml))
            {
                EmbedSignature(TempPdf, SignedPdf, "Sign", Encoding.ASCII.GetBytes(eSign.InnerHtml));
            }
            bytesFromPdf.InnerHtml = Convert.ToBase64String(GetBytesToSign(UnsngnedPdf, TempPdf, "Sign"));
        }

        public static byte[] GetBytesToSign(string unsignedPdf, string tempPdf, string signatureFieldName)
        {
            using (PdfReader reader = new PdfReader(unsignedPdf))
            {
                using (FileStream os = File.OpenWrite(tempPdf))
                {
                    PdfStamper stamper = PdfStamper.CreateSignature(reader, os, '\0');
                    PdfSignatureAppearance appearance = stamper.SignatureAppearance;
                    appearance.SetVisibleSignature(new Rectangle(200, 50, 0, 0), 1, signatureFieldName);
                    IExternalSignatureContainer external = new ExternalBlankSignatureContainer(PdfName.ADOBE_PPKMS, PdfName.ADBE_X509_RSA_SHA1);
                    MakeSignature.SignExternalContainer(appearance, external, 8192);

                    return SHA1.Create().ComputeHash(appearance.GetRangeStream());
                }
            }
        }

        public static void EmbedSignature(string tempPdf, string signedPdf, string signatureFieldName, byte[] signedBytes)
        {
            using (PdfReader reader = new PdfReader(tempPdf))
            {
                using (FileStream os = File.OpenWrite(signedPdf))
                {
                    IExternalSignatureContainer external = new MyExternalSignatureContainer(signedBytes);
                    MakeSignature.SignDeferred(reader, signatureFieldName, os, external);
                }
            }
        }

        private class MyExternalSignatureContainer : IExternalSignatureContainer
        {
            private readonly byte[] _signedBytes;

            public MyExternalSignatureContainer(byte[] signedBytes)
            {
                this._signedBytes = signedBytes;
            }

            public byte[] Sign(Stream data)
            {
                return _signedBytes;
            }

            public void ModifySigningDictionary(PdfDictionary signDic)
            {
            }
        }
    }