Making a PHP page that opens a HTML as if in a frame

I’m developing a theme and using Page Links To plugin to link to an old web 1.0 era guestbook of mine, which is still in use. The guestbook is an external html page. I want it to open so that my header and right sidebar remain visible, but that an additional left sidebar will appear, and the guestbook is in the center. I’m this far:

<?php get_header(); ?>

<div class="vieraskirjasidebar">

<ul>

    <li>Vieraskirjat  // This is a list of all my ancient guest books.
		<ul>
			joujoujou  // This is just a placeholder for the actual list items, which are not yet added.
		</ul>
	</li>
</ul>

</div>

<?php get_sidebar(); ?>

</div>
</body>
</html>

But now I somehow need to get the html file visible, too, and I have no clue how to do that. Thanks a thousand times for help!