Add Page Break button in WordPress Visual Editor

add this code in your theme/child theme functions.php. :

/* Add Page Break Button in First Row */
add_filter( 'mce_buttons', 'my_add_page_break_button', 1, 2 ); // 1st row

/*Add Page Break button in WordPress Visual Editor */
function my_add_page_break_button( $buttons, $id ){

    /* Only add this for content editor */
    if ( 'content' != $id )
        return $buttons;

    /* Add Page Break button after Insert Read More tag button */
    array_splice( $buttons, 13, 0, 'wp_page' );

    return $buttons;
}