If you know that you will want to access your references while using a mobile device, I recommend creating a shared folder and bookmarking the shared folder link. The resulting web page view is accessible on your iPad (or other mobile browser) even though the Paperpile app is not. This is useful, for instance, if you have a bunch of books you wish to look for in the library.
Unfortunately I don’t think you can add notes that are visible in this view, as it would be nice to be able to add something like call numbers in the notes field and see it in the shared folder web view.