Star PDF Watermark for Mac

Have a look at PDF Stamp, its not free but very easy to use . But costs $15.

I haven't played with this program too much, but what I've done with it I've like. The free version is actually pretty useful for just adding watermark text to PDFs. The only suggestions I have are that you can't manually change the angle; you have to use a "dial" and it's a bit of a crap shoot. The other is the placement of the text, it seems to be a bit off. All that being said, I actually sent feedback to the developers and they responded within 24 hours thanking for my feedback and responding directly to my suggestions. To me, this is what makes a good product great - the people behind it. I'm much more willing to overlook missing features and other minor gripes when I know the people who've developed it are actually listening and willing to do what they can to help.

The application is labeled ‘free’, even if not entirely correct. The free version is (very) limited. This is not correct, its creator should specify both versions. Otherwise, well, may be a useful tool sometimes.

There is a free version as well, limited to text watermark, no image. Quite good. Go to the developers site.