public void Context_Url_Rect()
        {
            var rect = new CGRect(10, 10, 100, 100);

            using (var url = new NSUrl(filename))
                using (var ctx = new CGContextPDF(url, rect)) {
                    ctx.BeginPage((CGPDFPageInfo)null);
                    ctx.SetDestination("xamarin", rect);
                    ctx.EndPage();
                }
        }