At first glance, it seems odd that Microsoft would give away one of its most lucrative products for free — but don’t worry, it all makes perfect business sense. For most of Microsoft’s life, a huge portion of its profits were derived from sales of the Office suite. Office, much like Windows, was the de facto productivity suite — it was installed on almost every home, school, and work machine. For the longest time, the various Office apps produced file formats that could only be viewed and edited by Office apps — and so it was almost impossible for alternatives to establish a beachhead. However, as the world’s reliance on Windows has diminished — as people move to smartphones, tablets, Chromebooks, and myriad other form factors — Microsoft has been forced to embrace other platforms, adopt open file formats, and generally reassess its strategy.In a rather surprising move, Microsoft has made the Office suite of apps — Word, Excel, and PowerPoint — free to download and use on the iPad and iPhone. Previously, an Office 365 subscription was required — but now, unless you’re a business customer or after some advanced functionality, you can now open and edit Office docs, spreadsheets, and slideshows for free on your iOS device. If all that wasn’t exciting enough, Microsoft also announced that the free Office suite is coming to Android, with beta access starting today. Both the iOS and Android versions of the Office suite have Dropbox integration baked in, following the announcement of a strategic partnership between the two companies earlier this week.

On the smartphone and tablet, where you can get reasonably good productivity apps for a few dollars (or free), Microsoft doesn’t have much choice than to price Office equivalently. One of the biggest strengths of Office is that, because it’s installed almost everywhere, you can fairly reasonably send a Word, Excel, or PowerPoint file to someone and expect that they can: a) open it, and b) probably edit it as well. As more and more of our time is spent on smartphones and tablets, Microsoft has to move quickly to ensure that Office remains dominant in the face of alternative, cheaper, disruptive apps — and thus, here we are, with free versions of the most excellent Office for iOS and Android.

While the Office apps are free for file viewing, editing, and creation, an Office 365 subscription — which is about $10 per month — is still required for some advanced functions, such as column formatting in Word or chart editing in Excel. You’ll also need an Office 365 subscription to open files from a business OneDrive or Dropbox account, which will ensure that corporate and enterprise customers don’t cancel their subscriptions any time soon. All round, this is a very savvy move from Microsoft to ensure the continued reign of Office — and at the same time, boosting its relevance in the mobile computing market.

Download Word / Excel / PowerPoint for iPhone and iPad — or sign up for the Office for Android preview