Are there events pub/sub for external application to subscribe to file/pdf uploads by wordpress admin

Use the action add_attachment, and look at the MIME type, then do whatever you need:

add_action( 'add_attachment', function( $post_id ) {
    
    if ( 'application/pdf' !== get_post_mime_type( $post_id ) ) {
        return;
    }
    
    // Here you can update a static file, send a HTTP 
    // request to the other site or send an email.
});