As you want to insert the button on only one/few specific pages you can simply create a shortcode[Reference] and insert it on specific page/pages.
Your code will be something like:
function hook_html(){
//function body
}
add_shortcode( 'dropbox_button', 'hook_html' );
And then just edit the specific pages and add the shortcode [dropbox_button] to them, you can also choose to add this in any section of the page that you want.