AceProject allows you to attach documents to a project (in which case the term Project Documents is used) as well as to specific tasks (which are called Task Documents in AceProject terminology).
When a normal user is denied the “Can open project” on a project, he/she is not able to access the left menu associated with the Project tab. Hence, the user is not able to reach the Project Documents area for that project. Additionally, this set up prevents the user from accessing tasks (and the Task Documents associated thereto) other than the ones he/she is assigned to or creator of.
So, to recap, if a normal user is not granted the “Can open project” right on a project, the user will only have access to Task Documents associated with assigned or created tasks in that given project. These tasks that the user may still access in the project will be available through the Home tab (for instance, through Home > Dashboard).