api.cs

[HttpPost]
public string ExportPDF(string document)
{
    byte[] bPDF;

    using (TXTextControl.ServerTextControl tx = new TXTextControl.ServerTextControl())
    {
        tx.Create();
        tx.Load(Convert.FromBase64String(document),
                TXTextControl.BinaryStreamType.InternalUnicodeFormat);

        X509Certificate2 cert = new X509Certificate2
            (Server.MapPath("~/App_Data/textcontrolself.pfx"),
             "yourpassword");

        TXTextControl.SaveSettings saveSettings = new TXTextControl.SaveSettings()
        {
            DigitalSignature = new TXTextControl.DigitalSignature(cert, null)
        };

        // save the document as PDF
        tx.Save(out bPDF, TXTextControl.BinaryStreamType.AdobePDFA, saveSettings);
    }

    // return as Base64 encoded string
    return Convert.ToBase64String(bPDF);
}