Preview emails and documents from SharePoint / Office 365
When using SharePoint Views, you also have the option to enable previews of the selected items.
Using Office Web App Preview (Server Side preview)
If an Office Web App preview server is available, previews will use the OWA server side technology where possible for selected file.
The Office Web App Server is available in Office 365, SharePoint 2013 or SharePoint 2010,
Using a local preview
In the event a file is not supported by the OWA preview technology or an OWA server has not been configured, a preview is rendered using a local preview technology. There are a number of Preview available for local previews to ensure maximum performance.