c# 修改pdf

PdfDocument doc = new PdfDocument();
            doc.LoadFromFile("wen.pdf");
            PdfPageBase page = doc.Pages[doc.Pages.Count-1];
            System.Drawing.Font font = new System.Drawing.Font("微软雅黑", 9, FontStyle.Regular);
            PdfTrueTypeFont trueTypeFont = new PdfTrueTypeFont(font, true);
            page.Canvas.DrawString("请在此输入建议：", trueTypeFont, new PdfSolidBrush(Color.Black), 80, 180);
            doc.SaveToFile("MyFirstPDF.pdf");
            //page.Canvas.DrawString("请在此输入建议：",
            //new PdfFont(PdfFontFamily.Helvetica, 9f),
            //new PdfSolidBrush(Color.Black), 80, 180);
            //doc.SaveToFile("MyFirstPDF.pdf");//如果用此中文乱码
            doc.Close();