Modify print page HTML

Yes you can.

What you can do is to copy print-posts.php from the plugin folder to your theme folder and edit it.

Sorry – I don’t see what I’m looking for in print-posts.php.
Where is the code for the print page header? i.e. where do the <meta> tags come from?
Where are <div class=”center”> and <div id=”Outline”> defined?
It’s the structure of this page I would like to change.
Thanks in advance.

Huh? The meta tags is there, example https://github.com/lesterchan/wp-print/blob/master/print-posts.php#L22
The styles for it is being defined in print-css.css

OK, got it now. Thanks.