static void Main(string[] args)
        {

            PdfCommon.Initialize();
            var pdfFilename = @"C:\............\doc20120514150242.pdf";
            for (int i = 0; i < 30; i++)
            {
                PdfToBitmap(pdfFilename);
                if (i % 5 == 0)
                {
                    Console.WriteLine($"Press any key to continue.");
                    Console.ReadLine();
                }
                   
            }


            Console.WriteLine($"Completed.");
            Console.ReadLine();
        }

        private static void PdfToBitmap(string pdfFilename)
        {
            Console.WriteLine($"Extracting images from pdf file: {pdfFilename}");

            var tfolder = @"C:\mytempfolder\pdfs";
            if (!Directory.Exists(tfolder))
                Directory.CreateDirectory(tfolder);

            /// <summary>
            /// Render whole PDF document using C# PDF Library
            /// </summary>
            using (var doc = PdfDocument.Load(pdfFilename)) // C# Read PDF Document
            {
                var pageIndex = 0;
                foreach (var page in doc.Pages)
                {
                    var dt = DateTime.Now;

                    var tfile = Path.Combine(tfolder, $"{Path.GetFileNameWithoutExtension(pdfFilename)}_{pageIndex}.png");

                    var imgObjects = ExtractImagesFromPage(page);

                    var text = page.Text;
     
                    var renderPage = true;
                    if (imgObjects.Count == 1 && text.CountChars == 0)
                    {
                        [h]var pdfBitmap = imgObjects[0].GetBitmap();

                        pdfBitmap.Image.Save(tfile, ImageFormat.Png);

                        pdfBitmap.Dispose();[/h]
                       
                        renderPage = false;
                    }

                    foreach (var item in imgObjects)
                        item.Dispose();


                    if (renderPage)
                    {
                        int width = (int)(page.Width / 72.0 * 300);
                        int height = (int)(page.Height / 72.0 * 300);
                        using (var bitmap = new PdfBitmap(width, height, true))
                        {
                            bitmap.FillRect(0, 0, width, height, FS_COLOR.White);
                            page.Render(bitmap, 0, 0, width, height, PageRotate.Normal, RenderFlags.FPDF_LCD_TEXT);

                            bitmap.Image.Save(tfile, ImageFormat.Png);
                        }
                    }

                    var msecs = (DateTime.Now - dt).TotalMilliseconds;
                    Console.WriteLine($"Page {pageIndex} Took {msecs} msecs");
                    pageIndex++;

                    page.Dispose();
                }

                doc.Dispose();
            }
        }

        private static List<PdfImageObject> ExtractImagesFromPage(PdfPage page)
        {
            var imageObjects = new List<PdfImageObject>();
            //Enumerate all objects on a page
            foreach (var obj in page.PageObjects)
            {
                var imageObject = obj as PdfImageObject;
                if (imageObject == null)
                {
                    imageObject.Dispose();
                    continue; //if not an image object then nothing do
                }
                imageObjects.Add(obj as PdfImageObject);
            }

            return imageObjects;
        }