When sending a copy of a project to a client, you may want to protect the file, so that it requires a password to print or edit. This will prevent your client from using it before you get paid!
To protect your file save it as a PDF (File > Save As… and choose Photoshop PDF). In the Save PDF dialog box, Click “Security” and under “Permissions,” you can enter a password to prevent printing and editing.
You can view the files in Acrobat, but you won’t be able to print it or copy it.
(This tip was part of my video, 5 Photoshop Tricks You Didn’t Know.)