Users can access presentations assigned directly to them or assigned to a group to which they belong.
When presentations are assigned to groups, the documents in the presentations must also be assigned to the groups. Otherwise, group members can access the presentation but cannot access its documents (unless they have individual user access).
To mitigate the risk of conflicting permissions, store presentations with the documents that they reference. Whenever possible, distribute documents and presentations to groups. It is easier to set permissions for all files in a folder and all users in a group than it is to manage permissions for individual files and users.