test.cs

public IActionResult Merge(string id)
{
    // url encoded
    id = id.Replace("%2F", "/");

    var templateData = Storage.GetTemplate(id);

    using (TXTextControl.ServerTextControl tx = new TXTextControl.ServerTextControl())
    {
        tx.Create();
        tx.Load(Convert.FromBase64String(templateData), TXTextControl.BinaryStreamType.InternalUnicodeFormat);
        
        using (MailMerge mailMerge = new MailMerge())
        {
            mailMerge.TextComponent = tx;
            mailMerge.MergeJsonData(InvoiceData.GetDummyInvoiceData());
        }
        
        tx.Save(out byte[] data, TXTextControl.BinaryStreamType.AdobePDF);

        // return pdf document
        return File(data, "application/pdf", "document.pdf");
    }
}