I am looking to show the users that have downloaded a document. This attribute

Take a look at the Content Security Utilities Plug-in available for download here in Appian Forum under the "Shared Components" record (forum.appian.com/.../all)- Smart Services.

This plug-in contains a custom function called GetDownloadersFromGroup() that returns the list of users who have downloaded the given documents and are part of the specified group. Useful to determine what users from a specific group have downloaded the any of the given files without going to the interface to check the download statistics.

Since the expression requires a group as the second argument, simple pass a constant of type group that points to an "All Users" group. Most likely you already have a group that holds all your users; if not, you can simply create one by using a group membership rule to add all users whose email contains @ forum.appian.com/.../Group_Management