Not all Microsoft Office products contain product keys. Microsoft Office's iOS and Android applications do not contain product keys, and its free Office Online software requires neither installation nor a product key.
Office Online and the iOS version of Microsoft Office both offer separate apps for Word, Excel and PowerPoint. Android's version of Microsoft Office bundles all three office applications into a single download called Microsoft Office Mobile.
Another component of the Microsoft Office suite, OneNote is available as a stand-alone download for both iOS and Android. It is also part of the Office Online suite. OneNote is free in all of these formats and does not use a product key.