And I'll even be nice and provide a simple PHP script to do this. First, put your common header into the file. Then, when you get to the point where the two differ, do this:

< ?php if($smalldoc==1) { ? >
Put your small stuff here
< ?php } else { ? >
Big stuff goes here
< ?php } ? >

Then put your common footer of the page here at the bottom. To switch back and forth, create a link to file.php?smalldoc=1 or just to the file.php. When the ?smalldoc=1 is in the URL, it should display the small format version.