If you've already saved your file, or haven't saved yet, click File>Save As... then you type your title, and below the first scroll down menu, there is a second one that says File Format. Click...
If you 'own' the file, you can change its status on the menu bar; if you are simply a reader of the file that someone else 'owns' and has sent to you, you may not change its modifiability, which has...