var aaa = this.viewer.Document.CreateBitmap(pageIndex, 1500);
            SaveImageToFile(aaa, @"C: \Users\wjx\Desktop\图书\b" + pageIndex + ".png");

        private void SaveImageToFile(BitmapSource image, string filePath)
        {
            BitmapEncoder encoder = GetBitmapEncoder(filePath);
            encoder.Frames.Add(BitmapFrame.Create(image));

            using (var stream = new FileStream(filePath, FileMode.Create))
            {
                encoder.Save(stream);
            }
        }

        /// <summary>
        /// 根据文件扩展名获取图片编码器
        /// </summary>
        /// <param name="filePath">文件路径</param>
        /// <returns>图片编码器</returns>
        private BitmapEncoder GetBitmapEncoder(string filePath)
        {
            var extName = System.IO.Path.GetExtension(filePath).ToLower();
            if (extName.Equals(".png"))
            {
                return new PngBitmapEncoder();
            }
            else
            {
                return new JpegBitmapEncoder();
            }
        }