When you upload a file from your computer to ArcGIS Online, if you share it with Everyone, you see a link to the file in the item details. If you don't share the file with Everyone, even if you share it with a Group in your organization, the url link in the item details page goes away. Is this by design? I found this help page:
which states: "You need to share the CSV and image files with everyone (public) to see the URL in the item details page."
In my case I'm sharing a pdf but I do see this behavior. The question is why? Maybe Esri doesn't want people using AGOL as a file repository?