Permissions can be set for a singular file in the Documents tab.

1. Either right-click a document and choose Permissions, or select a document by ticking it's checkmark, clicking More Actions, and selecting Permissions.
2. Click in the Permission column for the row of a role.
3. Select a permissions level: Hidden, View, Print, or Download. The Manage permission level can't be changed at the file level.
4. Click Save.

To verify, click the link in the window "Go to <filename> in Permissions Tab".