By default, AtoM does not allow public users to access the original upload file - the one exception being PDFs, since the small image thumbnails do not allow users to see all pages or read any textual content. However, this default is easily via the permission settings.
Navigate to Admin > Groups, and click on the "anonymous" group - these are the default permissions for public users.
In the Archival description permissions tab, enter edit mode and change the "View master" permission to "Allow," then save. Now public users should be able to click on the reference display image shown on related description view pages, and access the original upload file.