Difference between Navigation.PushModalAsync and new Navigation.PushModalAsync?

Dear Xamarin members,

I have a hard time with the Showing Pages/Navigation in Xamarin.Forms.

What is the difference between :
1) Navigation.PushModalAsync(new PDFViewer());
2) var page = new NavigationPage(new PDFViewer());
Navigation.PushModalAsync(page);

Regards,
Gert