From the Microsoft blog post: (my emphases)

Dropbox Inc. and Microsoft Corp. on Tuesday announced a strategic partnership to help people work more effectively. The two industry-leading companies are teaming up to integrate their services for collaboration across Dropbox and Microsoft Office on phones, tablets and the Web.

....This partnership will enable people at work, home and school to easily collaborate on documents, spreadsheets and presentations. These capabilities will first be available from within the Dropbox and Office apps on tablets and phones, and will soon be accessible via the Web.

Dropbox and Microsoft users can now do the following:

Access Dropbox from Office apps to get to their files and folders faster.

Edit Office files directly from Dropbox and sync them across devices.

Share new or edited files from the Office apps using simple Dropbox sharing functionality.

The functionality will first be included in the next updates to the Office apps for iOS and Android, coming in the next few weeks. The Web integrations between the Dropbox website and Office Online will be available in the first half of 2015. Dropbox will also make its application available on the Windows Phone and Windows tablet platforms in the coming months.