Hello Owen,
one option would be to add a parameter to the link URL, for example
<a href="http://yourdomain.com/mypage.php?pdf=1">Save to PDF</a>
and then render your page conditionally based on the presence of the pdf parameter.
The idea with the pdf parameter added to the URL is outlined in
this post, scroll down to the "Provide a PDF version of your web pages" section.