Most likely to conserve screen space, all of the formatting options are tucked into a Format bar, which you can access by hitting your device's Menu button. There are formatting options to bold, underline, or italicize text, and you can select from three different colors for text or when you want to highlight words or sentences in a doc. You can also make comments on shared Word documents. When you're finished, just save to SkyDrive, and your document will have your changes and be available to edit from your other devices.
In Excel you can tap on cells and pinch to zoom, just like you would expect. If you want to jump to a specific chart or go between sheets, there's a button up top, which pulls up a useful outline view. Next to that are a Search tool, an AutoFilter toggle (for filtering a subset of data within a range of cells), and a Formula button, which gives you access to a few simple, often-used formulas.
Just as on a desktop, you can enter formulas for cells, and -- when you edit a cell -- the values will automatically update in your spreadsheet. You also can quickly create a chart of selected data by highlighting fields, hitting your Menu key, and touching Create Chart. You have six different chart types to choose from, and the app will automatically create the chart in a new tab.
With PowerPoint, you can't create a new presentation like you can for Word and Excel. Still, you can view previously made presentations, make small edits, hide slides, and view and edit speaker notes so you can practice a presentation on the go. The app has a button at the top for quick navigation to other slides, or you can swipe horizontally to navigate to the previous or next slide. To get a feel for your presentation, you also can switch to landscape mode and swipe to view your slides in full screen. You won't be able to create new slides or perform more-complex actions like adding transitions, but the app will still be useful for smaller edits before a big meeting. Also, it's worth noting that the app doesn't let you move slides around within your presentation, while the iOS version does.