How to display a local PDF file that is not bundled in app

I've tried everything I can think of to display a PDF that my app downloads. I download it into the MyDocuments folder. I've tried using Xamarin.Forms webview. I've also created a custom renderer for iOS that does something like this:

        var viewer = UIDocumentInteractionController.FromUrl(NSUrl.FromFilename(TechDispatch.Views.PDFViewer._pdfPath));
        viewer.PresentOpenInMenu(new RectangleF(0, -260, 320, 320), this.View, true);

Nothing is working. I can find documentation for bundled resources, but haven't found anything for local content that is not bundled.