using Spire.Pdf;
    using System.Drawing;
    using System.Drawing.Imaging;

        /// <summary>
        /// 添加二维码到PDF
        /// </summary>
        /// <param name="sourcePdf">pdf文件路径</param>
        /// <param name="sourceImg">二维码图片路径</param>
        private void AddQrCodeToPdf(string sourcePdf,string sourceImg)
        {
            //初始化PdfDocument实例
            Spire.Pdf.PdfDocument doc = new Spire.Pdf.PdfDocument();

            //加载现有文档
            doc.LoadFromFile(sourcePdf);

            //获取第二页
            PdfPageBase page = doc.Pages[1];

            //加载图片到Image对象
            Image image = Image.FromFile(sourceImg);

            //调整图片大小
            int width = image.Width;
            int height = image.Height;
            float scale = 0.18f;  //缩放比例0.18f;
            Size size = new Size((int)(width * scale), (int)(height * scale));
            Bitmap scaledImage = new Bitmap(image, size);

            //加载缩放后的图片到PdfImage对象
            Spire.Pdf.Graphics.PdfImage pdfImage = Spire.Pdf.Graphics.PdfImage.FromImage(scaledImage);

            //设置图片位置
            float x = 516f;
            float y = 8f;

            //在指定位置绘入图片
            page.Canvas.DrawImage(pdfImage, x, y);

            //保存文档
            doc.SaveToFile(@sourcePdf);
            doc.Close();
            //释放图片资源
            image.Dispose(); 
        }