Dear Peeter,
Public folder access is folder based and can not defined in Access Level.
You need to create a group and assign "View" to the group on all public folders. Hence, any user who is part of this group can only View the content but can not modify. If you want view and refresh then use "View on Demand"
- First create a group in Users & Groups
- Go go Folders -> Manage -> Top Level Security -> All Folders
- Make Everyone - > No Access
- New Group -> View Access
Let me know if you face any issue.