Group members do not have permission to download pdf from a forum thread

A member of our community created a new forum thread and attached a downloadable pdf. The download link looks something like this:  

https://<COMMUNITY URL> /cfs-file/__key/communityserver-discussions-components-files/20/Tina-Bang-vocab-alphabetized.pdf

We have received multiple reports from group members that they do not have the permission to view it, but I have been able to download successfully as an Admin, regular user, and when I impersonate the members having the issue. We've reviewed all of our group and forum permissions and they appear to be set correctly. 

Any ideas on what the issue could be?