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This office suite from Google is free and is designed for working with Word, Excel, Powerpoint, and PDF documents. It requires a Google account and is tightly integrated with the Google Drive cloud storage.
Quickoffice is not the best office suite in this collection, but it's a solid performer for no cost. The spreadsheet functionality is good and can handle many Excel spreadsheets without problems.
Note that this suite was originally Quickoffice Pro HD until the company was bought by Google. The free Quickoffice app is the newest version released by Google and includes all functionality of the previous version.
This is a full-featured office suite that integrates word processing, spreadsheets, presentations, and PDF file handling. It can work directly with Microsoft Office documents, and is especially good at handling sophisticated documents.
The document editors in DocsToGo are very basic but may be all some professionals need.
Pages, Numbers, Keynote from Apple
This isn't an office suite, it consists of three separate apps from Apple. All are optimized for touch operation better than any other apps in this collection.
Pages is a word processor that makes it easy to create elaborately formatted documents. Numbers is a spreadsheet app that is designed to make the most of the iPad's screen. Keynote is a presentation app that can be used to create attractive presentations complete with animated effects. In typical Apple fashion all three apps have professional templates to simplify the building of new documents.
All three apps work with Apple's iCloud storage.
Apple recently created a stir with the announcement that these three apps are now free for buyers of new iOS and OS X devices.
Pages -- $9.99 (free on new iPads)
Numbers -- $9.99 (free on new iPads)
Keynote -- $9.99 (free on new iPads)