WordApplication.Open

Overload	Description
Open(System.IO.Stream)	Opens and returns an existing Document from the specified `Stream`.
Open(String)	Opens and returns an existing Document from disk.
Open(WordTemplate)	Takes a WordTemplate object representing a template Word file, and opens the file as a Document object.

Overload	Description
Open(Microsoft.SharePoint.SPDocumentLibrary, String)	Opens and returns an existing Document from a SharePoint Document Library.
Open(Microsoft.SharePoint.SPListItem, String)	Opens and returns an existing Document from a SharePoint List Item.