将GridView导出为PDF后，如何在iTextSharp中更改默认字体大小？

protected void btnExportPDF_Click(object sender, EventArgs e)
{
    Response.ContentType = \"application/pdf\";
    Response.AddHeader(\"content-disposition\", \"attachment;filename=GridViewExport.pdf\");
    Response.Cache.SetCacheability(HttpCacheability.NoCache);

    StringWriter sw = new StringWriter();
    HtmlTextWriter hw = new HtmlTextWriter(sw);

    GridView1.AllowPaging = false;
    GridView1.DataBind(); 
    GridView1.RenderControl(hw);

    StringReader sr = new StringReader(sw.ToString());
    Document pdfDoc = new Document(PageSize.A4, 10f,10f,10f,0f);
    HTMLWorker htmlparser = new HTMLWorker(pdfDoc);
    PdfWriter.GetInstance(pdfDoc, Response.OutputStream);

    pdfDoc.Open();
    htmlparser.Parse(sr);
    pdfDoc.Close();

    Response.Write(pdfDoc);
    Response.End();  
}