Lock PDFs and images
You can lock a PDF or image so that it can’t be edited or deleted. If you own a locked document, you can edit the document after unlocking it. Others can change the document’s contents only by creating a duplicate of the document and editing the duplicate.
Lock or unlock a PDF or image open in Preview: Move the pointer next to the document’s name in the title bar, click the arrow that appears, then select or deselect the Locked checkbox.
Tip: To lock or unlock a file in the Finder, select the file, choose File > Get Info, then select or deselect the Locked checkbox.