尝试以下方法检查URL是否返回nil:

Have a try with follow way to check whether URL return nil:

var documentURL = NSBundle.MainBundle.GetUrlForResource("Harvard", "pdf");
var pdfdoc = new PdfDocument(documentURL);

这是完整的示例代码:

var documentURL = NSBundle.MainBundle.GetUrlForResource("MyForm", "pdf");
if (documentURL != null)
{
    var document = new PdfDocument(documentURL);
    //var page = document.GetPage(0);

    var pdfview = new PdfKit.PdfView();
    pdfview.Frame = View.Frame;
    pdfview.Document = document;
    pdfview.AutoScales = true;
    pdfview.UserInteractionEnabled = true;
    pdfview.BackgroundColor = UIColor.Gray;

    View.AddSubview(pdfview);
}

效果: