How do I password protect my pdf files?

Use Adobe Acrobat. Open pdf file.

1.Click on Tools, protection, more protection, security properties.

2.On document properties - security method, click to change to PASSWORD SECURITY

3.Tick box, Require a password to open document, Type in your password.

and follow the prompts to complete.

NOTE if you want to change document permission too, passward cannot be the same.

Don't count on it as a form of protection, though. It's very easy to get around it and many PDF viewers don't enforce it at all, unfortunately.