Hey, you can use Adobe Acrobat to do this. Click Secure button and select encrypt with password. In pop-up window of Password Security- Settings, you can set the open password and owner password.
I found a article talking about how to create and crack pdf password in detail, ;)
Yes, I agree that you can hardly find freeware for Mac OS especially want to that office work. Online software may helps but it is a pity that you could not do that kind of work. As I am a Mac enthusiast, I would give my advice and hope you can take it for reference. There are three ways you may...