class PdfToPicture
    {
        /// <summary>
        /// 将PDF转换为图片
        /// </summary>
        /// <param name="pdfPath">pdf文件位置</param>
        /// <param name="pageNumber">pdf文件张数</param>
        /// <param name="size">pdf文件尺寸</param>
        /// <param name="outputPath">输出图片位置与名称</param>
        public void RenderPage(string pdfPath, int pageNumber, System.Drawing.Size size, string outputPath, int dpi = 300)
        {
            using (var document = PdfiumViewer.PdfDocument.Load(pdfPath))
            using (var stream = new FileStream(outputPath, FileMode.Create))
            using (var image = GetPageImage(pageNumber, size, document, dpi))
            {
                image.Save(stream, ImageFormat.Jpeg);
            }
        }
        private static Image GetPageImage(int pageNumber, Size size, PdfiumViewer.PdfDocument document, int dpi)
        {
            return document.Render(pageNumber - 1, size.Width, size.Height, dpi, dpi, PdfRenderFlags.Annotations);
        }
    }

 static void Main(string[] args)
        {
            var pdf = PdfDocument.Load(@"E:\BBBBBPDF\sample.pdf");
            var pdfpage = pdf.PageCount;
            var pagesizes = pdf.PageSizes;

            PdfToPicture p2p = new PdfToPicture();
            for (int i = 1; i <= pdfpage; i++)
            {
                Size size = new Size();
                size.Height = (int)pagesizes[(i - 1)].Height;
                size.Width = (int)pagesizes[(i - 1)].Width;
                p2p.RenderPage(@"E:\BBBBBPDF\sample.pdf", i, size, @"E:\BBBBBPDF\sample" + i + @".jpg");
            }
            Console.WriteLine("Success");
            Console.Read();
        }