Have you tried:
var image = iTextSharp.text.Image.GetInstance(page, ImageFormat.Png);
image.SetAbsolutePosition(0, 0);
image.ScaleAbsoluteHeight(_doc.PageSize.Height);
image.ScaleAbsoluteWidth(_doc.PageSize.Width);
_directContent.AddImage(image);
_doc.NewPage();
It works well for me.