iTextSharp操作表格排版问题

using iTextSharp.text;
using iTextSharp.text.pdf;
using System.IO;

namespace testPdf
{
    class Program
    {
        static void Main(string[] args)
        {
            BaseFont BF_Light = BaseFont.CreateFont(@"C:\Windows\Fonts\simsun.ttc,0", BaseFont.IDENTITY_H, BaseFont.EMBEDDED);
            Document doc = new Document();

            PdfWriter writer = PdfWriter.GetInstance(doc, new FileStream(string.Format("Demo.pdf"),FileMode.Create));
            doc.Open(); 
            Paragraph p = new Paragraph("标题\n\n",new Font(BF_Light,18));
            p.Alignment = 1;//居中
            doc.Add(p);

            p = new Paragraph("第一个单元格", new Font(BF_Light,12));//测试再次调用P

            PdfPCell cell = new PdfPCell(p);
            cell.HorizontalAlignment = 1;//设置单元格内居中
            //cell.VerticalAlignment = 1;
            int[] widths = { 5, 10 };
            PdfPTable dt1 = new PdfPTable(2);
            dt1.SetWidths(widths);//设置每列的宽度（求和百分比5,10=1,2）
            cell.Rowspan = 2;//设置跨度行数（Colspan设置跨度列数）
            dt1.AddCell(cell);
            dt1.AddCell(new PdfPCell(new Paragraph("第二个单元格", new Font(BF_Light, 12))));
            dt1.AddCell(new PdfPCell(new Paragraph("第三个单元格", new Font(BF_Light, 12))));
            doc.Add(dt1);

            PdfPCell cell3 = new PdfPCell(new Paragraph("最后一个", new Font(BF_Light)));
            PdfPTable dt3 = new PdfPTable(2);
            dt3.AddCell(cell3);
            dt3.WidthPercentage = 85;
            PdfPCell cell4 = new PdfPCell(new Paragraph("再加一个", new Font(BF_Light)));
            dt3.AddCell(cell4);

            doc.Add(dt3);
            doc.Close();
        }
    }
}