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If you have purchased Pages for the iPad, you can install it for free on all your iPads, iPods touch and iPhones, but you have to purchase Pages for Mac because your Mac uses a different operating system that can't run iPad applications.
To purchase Pages, open App Store and purchase it. If you don't have the Mac App Store on your Mac, you will have to purchase iWork '09 on a disc
Pages purchased from the Mac App Store can be installed on all Macs using the same Apple I.D.
Pages purchased as part of iWork demo or download with serial number, or on DVD can only be installed on one Mac unless it is a Family licence in which case it can be installed on 5. But this is an honesty system, relying on you to do the right thing.