[Plugin: article2pdf] Install help

Hi, sorry to be a noob but I need some help with this:

If you use non standard permalinks (e.g. “/%postname%”), add the link html code PDF Version to your theme. I put the html code into the “single.php” and the “page.php” of my theme and it works fine for me.
Example link: https://my-blog.org/my-category/my-post/?article2pdf=1

Where in the files do we add the link?