Lock PDFs and images in Preview on Mac
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.
In the Preview app on your Mac, open a PDF or image that you want to lock.
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.