The plugin's HTML output is wrong because it uses an ID instead of a class in order to identify the link:
<a href="http://example.com/?p=10&output=pdf"> My Text</a>
So, if you have the pdf link showing within a loop, there are multiple IDs with the same value "pdfbutton".
The solution is to convert the id to a class (class="pdfbutton") or even better, converting it to a class and adding an unique id:
class="pdfbutton" id="pdfbutton-XX", where XX is the post ID.
As a side note: It would be useful for us to have a github repo for the plugin so we can fork, edit and push-request the code, that way we can contribute to it too.
Thanks for the plugin.