test.cs

[HttpPost]
public string MergeDocument(string Document)
{
    using (TXTextControl.ServerTextControl tx = new TXTextControl.ServerTextControl())
    {
        tx.Create();
        tx.Load(Convert.FromBase64String(Document), 
                TXTextControl.BinaryStreamType.InternalUnicodeFormat);

        using (TXTextControl.DocumentServer.MailMerge mailMerge = 
               new TXTextControl.DocumentServer.MailMerge())
        {
            mailMerge.TextComponent = tx;
            mailMerge.FieldMerged += MailMerge_FieldMerged;

            string data = System.IO.File.ReadAllText(Server.MapPath("~/App_Data/data.json"));
            mailMerge.MergeJsonData(data, false);
        }

        byte[] results;

        tx.Save(out results, TXTextControl.BinaryStreamType.InternalUnicodeFormat);
        return Convert.ToBase64String(results);
    }
}