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You can now edit files that you've saved locally (these were previously read-only), and you can't buy Drive storage as an in-app purchase on iOS devices from either Docs or Sheets, though it's still an option in the Drive app. Certain features have been moved around, but as of this writing there are no extra formatting features in the standalone Docs and Sheets apps that weren't already available in Drive, though there are small improvements.
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The Docs and Sheets apps that have been released mostly duplicate functionality previously available in the Google Drive app. Slides, an app for creating and editing presentations, is "coming soon." The apps are compatible with iPhones and iPads running iOS 7 and Android phones and tablets running version 4.0 or newer. To view and edit any documents you created in the Web version of Drive on your phone or tablet, you've always needed to install and use the Google Drive app, but Google has now altered course a bit: the company just released new, standalone Google Docs and Google Sheets productivity apps for Android and iOS, breaking document and spreadsheet editing out from the Drive app for the first time on mobile devices.
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Drive competes with Dropbox, OneDrive, and other cloud services, but at the time it also consumed Google Docs, the company's Web-based productivity suite. Google announced its Drive cloud storage service just over two years ago, if you can believe it.
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