How do I view a page so that the whole page fits in my open area .

[MooGooGai]

On the view size I would like a page to be sized to my viewing area. When I
switch to another page and come back, often the page has been resized
automatically back to 100%, so I have go up and select "page" under view
size. Can I set it so that this doesn't happen: so the last zoom size I
viewed a page is kept?

Also I would like this to be a keyboard command to set vaiious page size
zooms (100%, fit page, etc.) so I don't need to use the toolbar.

Thanks, in advance, for any ideas/help.