PdfCommon.Initialize()
using (var doc = PdfDocument.Load(@"d:\0\test_big.pdf"))
{
    int dpi = 96;
    foreach (var page in doc.Pages)
    {
        int width = (int)(page.Width / 72.0  * dpi);
        int height = (int)(page.Height / 72.0 * dpi);
        using (var bitmap = new PdfBitmap(width, height, true))
        {
            bitmap.FillRect(0, 0, width, height, Color.White);
            page.Render(bitmap, 0, 0, width, height, PageRotate.Normal, RenderFlags.FPDF_LCD_TEXT);
            bitmap.Image.Save(...);
        }
        page.Dispose();
    }
}