Firstly, you would need that "Download as PDF" psuedo selector on the .btn-secondary selector part, not on the display:none rule.
However, you should probably look to see if the plugin that is displaying the button is wrapping all of it's content in another class or id further up the DOM so that you could use something like
.myplugin .btn-secondary {display:none;}
However, you should probably look to see if the plugin that is displaying the button is wrapping all of it's content in another class or id further up the DOM so that you could use something like
.myplugin .btn-secondary {display:none;}