test.cs

using TXTextControl.DocumentServer.Fields;

using (TXTextControl.ServerTextControl tx = new TXTextControl.ServerTextControl())
{
    tx.Create();

    TXTextControl.LoadSettings ls = new TXTextControl.LoadSettings()
    {
        ApplicationFieldFormat = TXTextControl.ApplicationFieldFormat.MSWord,
        LoadSubTextParts = true
    };

    tx.Load("template.docx", TXTextControl.StreamType.WordprocessingML, ls);

    using (TXTextControl.DocumentServer.MailMerge mailMerge =
      new TXTextControl.DocumentServer.MailMerge())
    {
        var jsonData = System.IO.File.ReadAllText("data.json");

        mailMerge.TextComponent = tx;
        mailMerge.MergeJsonData(jsonData);
    }

    tx.Save("output.pdf", TXTextControl.StreamType.AdobePDF);
}