As a little experiment I signed up with adwords and placed a listing, just in the UK. Google rejected my first ad for trademark infringement, so the revised ad ran for about 9 hours, from around midday. The search terms I used were free mac ocr pdf.
In that time the ad was shown 3,445 times, and got 3 clicks, all related to the search term 'free'. Looking at analytics it seems that these clicks were actually searching for 'free games' - obviously VelOCRaptor is such an attractive title that the respondants ignored the words in the ad.
What surprised me is that Google doesn't seem prefer to run the ad when more than one of the terms is matched. In fact, in all the times I've searched for 'free mac ocr pdf' it's never been shown to me. Given this, 'free' and 'mac' are rubbish keywords, as they trigger (infrequently, as they must be popular) when someone is looking for 'free holiday' or 'mac games'.
So I've changed my strategy and am looking at phrases. I now use 'image to text' 'mac ocr' and 'ocr to pdf'. I figure that these will be shown less frequently, but with far better targetting.