使用itextsharp进行html转pdf时生成的pdf里不显示checkbox复选框问题。

使用itextsharp进行html转pdf时生成的pdf里不显示checkbox复选框，请问一下这个问题该怎么去解决？

这个是转换的代码，保存到根目录里的文件夹

     public static string ConvertHtmlToPdf(string html, string path, int? NotA4 = null)
    {
        string url = string.Empty;
        byte[] data = Encoding.UTF8.GetBytes(html.ToString()); //字串转成byte[]
        MemoryStream outputStream = new MemoryStream();//要把PDF写到哪个串流
        MemoryStream msInput = new MemoryStream(data);
        Document doc;
        //参数为空，默认A4 竖向，否则横向
        if (NotA4 != null)
            doc = new Document(new RectangleReadOnly(PageSize.A4.Height, PageSize.A4.Width)
            {
            });
        else
            doc = new Document();

        doc.SetMargins(20f, 20f, 20f, 0f);
        //要写PDF的文件，建构子没填的话预设直式A4
        PdfWriter writer = PdfWriter.GetInstance(doc, outputStream);
        //指定文件预设开档时的缩放为100%
        PdfDestination pdfDest = new PdfDestination(PdfDestination.XYZ, 0, doc.PageSize.Height, 1f);
        //开启Document文件 
        doc.Open();
        ////使用XMLWorkerHelper把Html parse到PDF档里
        XMLWorkerHelper.GetInstance().ParseXHtml(writer, doc, msInput, Encoding.UTF8);
        PdfAction action = PdfAction.GotoLocalPage(1, pdfDest, writer);
        writer.SetOpenAction(action);
        doc.Close();
        msInput.Close();
        outputStream.Close();
        if (outputStream.ToArray().Length > 0)
        {
            var array = outputStream.ToArray();
            FileStream fs = new FileStream(path, FileMode.Create);
            //将byte数组写入文件中
            fs.Write(array, 0, array.Length);
            fs.Close();
            url = path;
        }
        return url;
    }