You don't need WebApp Builder for that. But you can later. Stick with me a sec:
Let's say you have a layer of points, and each is associated with a scan. Maybe each is a PNG, or JPEG, or PDF. You can create a hosted feature layer for the points, and the scan as an attachment for each point. Or if your scan files are sitting on a web server somewhere, you can put the URL to the file in a column in that layer's attribute table. Either way, then open the Map Viewer, bring that layer in, and configure the popup to give users interactive access to those scans by clicking on each point.
Save that out as a web map. At that point you can share the map as-is. It even gives you an arcg.is short URL for the map if you want. If all you want to do is share a map with a layer that users can click on to open associated scans, maybe you're done.
But then.... if you want to use WebApp Builder to wrap a custom web app around that web map, it'll work the same, but then you have a lot more options for configuring and styling the app, as well as choosing from dozens of tools/widgets to put on the app for your end users to use with your map.
Everything described above has pretty good step-by-step tutorials on the web site. First time might be a little klunky, but the learning curve isn't steep.
