A File menu is available in the Library Explorer and in a Book object. Its functionality depends on where it's located and the role of the user.
File menu in Library Explorer
In the Library Explorer, the options available in the menu depends on whether you are a Security Administrator or a User.
File Menu in the Library Explorer for a Security Administrator
File Menu in the Library Explorer for a User
File Menu in a Book Object
The File menu in a Book object provides three save options - Save Topic (save the contents of the current topic), Save Book (save the Table of Contents structure of the current book), or Save All (save both the content topic and the current book).
File Menu in the Book with Save options