When you install Office Mobile on an iPhone, you go through a quick setup process that closely resembles setup for the desktop apps.

The Office Mobile app requires an iPhone 4 or later (or an iPod Touch Fifth generation), running iOS 6.1 or later. This version of Office Mobile was specifically designed for the small screen sizes of an iPhone; Microsoft declined to comment about its plans for an iPad-specific version. For now at least, the company recommends that iPad users stick with the Office Web Apps.

The new Office Mobile app is free, although that slender price tag comes with a catch: You need an Office 365 subscription to activate the software. (If you don’t have an Office 365 subscription, you can sign up for a free trial on the Web or use the in-app purchase option in the Office Mobile app.) Any Office 365 subscription plan that includes the Office 2013 software will allow you to install and activate Office Mobile on up to five iOS devices.

The four icons at the bottom of the main Office screen give you access to documents you've created or opened recently, as well as options for browsing folders in cloud storage.

Office Mobile includes three apps: Word, Excel, and PowerPoint. (OneNote is already available as a separate app for the iPhone.) You can create new Word documents and Excel spreadsheets (but not PowerPoint presentations) directly in the app, and then save them to the cloud using SkyDrive, SkyDrive Pro, or SharePoint. You can view documents in any of the three formats and do most light editing tasks.

In a PowerPoint presentation, the slide organizer view lets you see a presentation at a glance and rearrange slides by dragging slides into position.

One of the three icons at the top of any Office Mobile window allows you to switch into editing mode, where you can choose from tools that are appropriate for the document type you're working with.

Any Office Mobile document can be viewed or edited in landscape mode. That's especially useful for PowerPoint slides. Tap any of these slides to begin an impromptu slide show. 

When you select PowerPoint's Editing Tools in Office Mobile, you switch to a clean screen where you can add and edit text directly. Click Done to automatically save your changes.

Office Mobile automatically reformats Word documents so they're readable on a small screen. You don't have access to the full assortment of reviewing options, but you can add a comment to any selection.

When you select some text and tap the Format button, this abbreviated set of options appears. You can't change fonts or select a specific font size, but you can increase or decrease the font sizes with a tap. You can also change font attributes and highlight text easily.

Office documents attached to email messages appear in the iPhone Mail app as a preview. To open them directly, tap the button in the top right corner and choose the Open in Office Mobile option.

Excel spreadsheets automatically reformat to accommodate the small iPhone screen. The result can be surprisingly readable, as shown here. Note that this workbook includes multiple tabs, which are available with a single tap.

The Editing options for Excel worksheets in Office Mobile include the ability to create a quick chart, although customization options are somewhat limited.

One of the sticking points in the negotiations between Microsoft and Apple over Office on iOS reportedly involved the in-app purchase options. Surprisingly, Microsoft has included the option to purchase a one-year Office 365 Home Premium subscription directly from within the app. If you choose this option, Apple gets its 30 percent cut.