test.cs

private void menuItemLoad_Click(object sender, EventArgs e)
{
    // set filter to tx and pdf
    openFileDialog1.Filter = "TX Text Control Documents (*.tx)|*.tx|PDF Documents (*.pdf)|*.pdf";

    if (openFileDialog1.ShowDialog() == DialogResult.OK)
    {
        // load the document into a byte array
        var bytes = File.ReadAllBytes(openFileDialog1.FileName);
        // get the filename
        string filename = Path.GetFileName(openFileDialog1.FileName);
        // convert the byte array to a Base64 string
        var stringBase64 = Convert.ToBase64String(bytes);
        // create a JavaScript string that calls the loadDocument() function
        string loadSettings = "{ pdfjs: { basePath: 'https://cdnjs.cloudflare.com/ajax/libs/pdf.js/3.2.146' } }";
        string js = "TXDocumentViewer.loadDocument('" + stringBase64 + "', '" + filename + "', null, " + loadSettings + ");";
        // execute the JavaScript string
        webView21.ExecuteScriptAsync(js);
    }
}