If enabled, a preview of Office documents will be automatically generated. Note that this only apply to files uploaded after the setting is enabled. However, this allows then to click on the thumbnail of the file, and browse through a small preview of the entire document. In addition, for users that use a modern browser, these documents can then be viewed in the online reader (users with a supported browser will see a Reader icon instead of a View icon for these files).
To enable go to Admin > Settings, scroll down to resource section and click on the "Convert for Full Preview".