[System.Web.Services.WebMethod]
    public static void converttopdf(string HTMLString)
    {

        string fileLocation = Path.GetTempPath() + "\" + "_specod.pdf";

        TextReader reader = new StringReader(HTMLString);
        Document document = new Document(PageSize.A4, 30, 30, 30, 30);

        PdfWriter writer = PdfWriter.GetInstance(document, new FileStream(fileLocation, FileMode.Create));
        HTMLWorker worker = new HTMLWorker(document);

        document.Open();
        string ttf = Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.Fonts), "arial.ttf");
        iTextSharp.text.FontFactory.Register(ttf);

        iTextSharp.text.html.simpleparser.StyleSheet ST = new iTextSharp.text.html.simpleparser.StyleSheet();
        ST.LoadTagStyle(HtmlTags.BODY, HtmlTags.ENCODING, BaseFont.IDENTITY_H);

        worker.SetStyleSheet(ST);
        worker.StartDocument();
        worker.Parse(reader); // Здесь исключение

        worker.EndDocument();
        worker.Close();
        document.Close();
    }