So I just ran into this, with the "View certificate" Button and it was getting frustrating for our end users. I ended up editing the code and I added this line to make it happen:
$downloadbutton = '<br><a href="/mod/customcert/view.php?id='.$cm->id.'&downloadown=1" class="btn btn-primary" target="_blank">View certificate</a>';
Instead of using the render function, just built the HTML and added it in there for the time being.
Thanks.