test.cs

[HttpGet]
[Route("Home/LoadDocument")]
public IActionResult LoadDocument()
{
    byte[] document;

    using (var tx = new TXTextControl.ServerTextControl())
    {
        tx.Create();
        tx.Load("App_Data/invoice.tx", TXTextControl.StreamType.InternalUnicodeFormat);

        var jsonData = System.IO.File.ReadAllText("App_Data/data.json");

        using (var mailMerge = new MailMerge { TextComponent = tx })
        {
            mailMerge.MergeJsonData(jsonData, true);
        }

        tx.Save(out document, TXTextControl.BinaryStreamType.InternalUnicodeFormat);
    }

    return new JsonResult(document);
}