Word Web App cannot open this document due to an unknown error. If the problem persists, try opening the file in Microsoft Word.
Problem : This error usually occurs when you have not configured the service applications after you have installed the office web application package on your SharePoint Farm.
Resolution : Log into your SharePoint Central Administration site, navigate to your service application pages and create a service application for Word, PowerPoint and Excel. Once complete perform a IISRESET (not necessary) and try and re-open your document using Office Web Applications.
I hope this resolution has worked for you as it worked for me.