If you asked someone on the street to name a Microsoft product they can’t live without, they would likely mention Microsoft Office, the suite that includes Outlook, Word, Excel and PowerPoint. Even Apple fans find themselves installing Office the second they buy a new Mac. IPhone owners have yearned for a way to access and edit Office documents on the go, yet Microsoft has kept this valuable asset restricted to its Windows Phones and Surface tablets.

Until now.

Last Friday, Microsoft released Office Mobile, a free app in the Apple App Store. This mobile version of Office lets you work on something at your desk, like a Word document or PowerPoint presentation, leave your desk and pull up the exact same document on your iPhone later. Any changes you make get saved back to a copy of the document and are there when you open it at your desk again.

This is a significant step for Microsoft, which has watched many of its users, especially younger users, migrate to free cloud-based programs like Google Drive, or to mobile office suites like Quickoffice. I’ve relied heavily on creating and sharing documents with other Google users via Drive for projects like planning my wedding, organizing To Do lists with my husband and coordinating my sister’s baby shower.

Unfortunately, there are many catches to using Office Mobile on the iPhone. If you can get past these, it’s a solid app that does a good job of making you forget you’re working on a small screen. It uses a lot of white space without excess text that would create clutter and its minimal number of icons allows easy access to actions like viewing and editing.

The first thing people should know is that Office Mobile only works for people who have a Microsoft Office 365 account. This cloud-centric, subscription version of Office starts at $80 a year for students using Office 365 University or $100 a year for Office 365 Home Premium users. If you only use a more traditional, desktop-based version of this suite, like Office 2011 or even Office 2013, you can’t use Office Mobile.

Second, it isn’t an iPad app, though you can hit the “2x” magnification button on your iPad to see it in a tablet-sized view with some slight pixilation. The iPhone’s 4-inch screen isn’t too small to use for creating, reading or editing Word documents, but cells of numbers and text in Excel spreadsheets aren’t exactly ideal for the iPhone’s screen.

Third, Office Mobile isn’t available for Android, so anyone who owns, say, a Samsung Galaxy S III or HTC One can’t use this app.

And there are other caveats. Office Mobile for iPhone doesn’t include Outlook, so if you’re a big fan of this email program, you’re out of luck on the go. Also, you can’t create PowerPoint presentations from your iPhone—though you can access and edit Word, Excel and PowerPoint.

One purchase of Office 365 allows up to five installations on Windows PCs or Macs and up to five installations on phones, not including Windows Phones, which come preloaded with a more extensive version of Office Mobile. But even though I tested Office Mobile on an iPhone and an iPad, my account only reflected my computer installations of Office 365. A spokesman for Microsoft said this will be updated in the future to show a more comprehensive list of installations.

I signed into my Office 365 account and found all of the documents I saved to SkyDrive waiting for me in this app. (SkyDrive is the name for Microsoft’s cloud-storage system and it synchronizes documents so they reflect recent changes no matter where you open them.) Unlike some competing iPhone office suites, the app only saves documents to SkyDrive, not to the phone itself or other cloud services.

Four quick access buttons get you started with this app: Recent, Open, New and Settings. The Recent panel groups Word, Excel and PowerPoint documents together, yet sorts them into helpful time-related sections like Today, Yesterday, Two Weeks Ago and Older. The Open panel gives you access to SkyDrive or to a SharePoint account, which is Microsoft’s more corporate-focused option for cloud storage. In the New panel, I saw templates like Agenda and Outline for Word and Budget and Mileage Tracker for Excel. Templates like these are especially helpful if you’re creating a document on the go using a small screen like the iPhone.

Within projects in Word and Excel, I could tap an eye icon in the top right to change to Outline View or to search for a specific word in the document. Text formats can be adjusted, including highlighting, font size, strikethroughs, bolding and others, but you can’t change a font type. In Excel, AutoSums can be added, charts can be created and cells can be formatted. In a PowerPoint presentation, I edited slide text and browsed many slides at once in the Presentation View. Turning my iPhone into landscape view showed a slide taking up the full screen, while portrait view displayed my speaker notes below the slide—a handy cheat sheet for presentations.

As I accessed documents, any comments I made on them were noted in a small red tab. Tapping on this tab also gave me access to comments from others with whom I shared the document. New projects and edits to existing projects aren’t automatically saved as you go. Rather, when you navigate away from the document, you’re prompted to save changes to your SkyDrive, or simply discard changes.

Office Mobile for iPhone works well—if you meet all of the qualifications to use it. But Microsoft needs to demonstrate its presence on more platforms, including iPads and Android devices, to lure Office users from the many available free alternatives.

Email katie.boehret@wsj.com