Link: <http://www.example.com/downloads/white-paper.pdf>; rel="canonical"

RewriteRule ([^/]+).(pdf|doc|txt)$ - [E=FILENAME:$1]
<FilesMatch ".(pdf|doc|txt)$">
    Header add Link '<http://www.example.com/poets/name-surname/poems/%{FILENAME}.htm>; rel="canonical"'
</FilesMatch>

<?php
 header('Link: <http://www.example.com/poets/name-surname/poems/'.$poemid.'poem-title.htm; rel="canonical"');
 ?>