using (var doc = PdfDocument.Load(@"document.pdf"))
{
	var page = doc.Pages[0];
	int width = (int)page.Width;
	int height = (int)page.Height;
	int stride = width + (4 - width % 4);
	IntPtr buffer = Marshal.AllocHGlobal(stride * height * 1);
	using (var bmp = new PdfBitmap(width, height, BitmapFormats.FXDIB_8bppRgb, buffer, stride))
	{
	        bmp.FillRect(0, 0, (int)width, (int)height, Color.White);
                page.Render(bmp, 0, 0, width, height, PageRotate.Normal, RenderFlags.FPDF_GRAYSCALE);
		bmp.Image.Save(@"document.png", ImageFormat.Png);
	}
}