Users can access presentations assigned to them or the groups to which they belong.
Both presentations and the documents within them must be assigned to groups. Otherwise, group members can access the presentations but not the documents within them (unless granted individual access).
To mitigate the risk of conflicting permissions, store presentations and their documents in the same folder. 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 to manage permissions for individual files and users.