Ask HN: Solutions for editing Microsoft Word docs in webapp?

I'm looking for a solution that allows users of our SAAS webapp to edit their uploaded Microsoft Word documents within our webapp.

Dropbox seems to have achieved this by integrating with Microsoft Office Online [1]. However, it isn't clear to me whether Microsoft supports this for any old SAAS app, or whether it's only for internal business use-cases where each user has an Office365 subscription.

Has anyone achieved this integration with their webapp? Alternatively, are there any other SAAS integrations that would allow users to edit their uploaded Word docs online?

[1] http://wopi.readthedocs.io/en/latest/index.html