convert pdf to image c#

static void Main(string[] args)
        {
            PdfDocument doc = new PdfDocument();
            doc.LoadFromFile(@"D:\test.pdf");
            List<string> list = new List<string>();
            for (int i = 0; i < doc.Pages.Count; i++)
            {
                System.Drawing.Image bmp = doc.SaveAsImage(i);
                string fileName =string.Format("Page-{0}.png", i + 1);
                bmp.Save(fileName, System.Drawing.Imaging.ImageFormat.Png);
                list.Add(fileName);
            }
            Bitmap b = CombineBitmap(list.ToArray());
            b.Save("D:\\total.png");
            foreach (var item in list)
            {
                File.Delete(item);
            }
        }
        public static System.Drawing.Bitmap CombineBitmap(string[] files)
        {
            //read all images into memory
            List<System.Drawing.Bitmap> images = new List<System.Drawing.Bitmap>();
            System.Drawing.Bitmap finalImage = null;

            try
            {
                int width = 0;
                int height = 0;

                foreach (string image in files)
                {
                    //create a Bitmap from the file and add it to the list
                    System.Drawing.Bitmap bitmap = new System.Drawing.Bitmap(image);

                    //update the size of the final bitmap
                    width += bitmap.Width;
                    height = bitmap.Height > height ? bitmap.Height : height;

                    images.Add(bitmap);
                }

                //create a bitmap to hold the combined image
                finalImage = new System.Drawing.Bitmap(width, height);

                //get a graphics object from the image so we can draw on it
                using (System.Drawing.Graphics g = System.Drawing.Graphics.FromImage(finalImage))
                {
                    //set background color
                    g.Clear(System.Drawing.Color.Black);

                    //go through each image and draw it on the final image
                    int offset = 0;
                    foreach (System.Drawing.Bitmap image in images)
                    {
                        g.DrawImage(image,
                          new System.Drawing.Rectangle(offset, 0, image.Width, image.Height));
                        offset += image.Width;
                    }
                }

                return finalImage;
            }
            catch (Exception)
            {
                if (finalImage != null)
                    finalImage.Dispose();
                //throw ex;
                throw;
            }
            finally
            {
                //clean up memory
                foreach (System.Drawing.Bitmap image in images)
                {
                    image.Dispose();
                }
            }
        }