Since you are using Pro, no need for custom CSS to customize button. We can achieve button style on advance settings available on the element. See Interactions from here:
Your setup with the pdf viewer is correct just linking to the page that should display the PDF. The issue is why the page doesn't render PDF correctly. From your shortcode, I can see you are using this plugin: https://wordpress.org/plugins/pdfjs-viewer-shortcode/. Can you share your site credentials on a secure note so we can see your setup. If the issue is coming from the plugin, it would be better to consult plugin developer.