Saving document to server in Java

We have an existing web application written in Java. We want to give our users the ability to open a Word document that is stored in our application on the server. So when the user downloads the document from our server, Word pops up and they then can edit this document on the client. Once finished editing the document we would like to give them the ability to save this document automatically back to the server where they retrieved it, by simply selecting on the Word Save icon. Similar to how SharePoint does it.

Seems to me SharePoint would be a good solution for this and they do it very well, but unfortunately we cannot use SharePoint in our architecture.

There also seems to be a lot of server-side possibilities that we can use to upload and open word documents from our server, I feel we can solve these use cases easily.

My big problem is the automatic saving of an edited document by the client back to our server where the document was downloaded

I am not Microsoft proficient to know what the best solution would be.

1. Should we write a Word AddIn to save to our server?
2. How do we get the Word AddIn into our users desktops?
3. Can we embed a Word AddIn into the Word document that is opened on the client?