OS X El Capitan: Lock a document
To protect documents from being accidentally changed or deleted, you can lock them. When a document is locked, you can move it to the Trash, but you can’t empty the Trash with the document in there.
Lock a closed document: Select it, choose File > Get Info, then select Locked.
Lock an open document: Hold the pointer to the right of the document title, click the arrow, then select Locked.
To unlock a document, deselect Locked.
To protect folders and disks, set permissions for them.
Last Modified: Apr 13, 2016