Add a pdf attachment to WooCommerce completed order email notification

There are some errors in your code: The $email_object function argument is the wrong variable name and should be instead $order to match with your first if statement.

Now for the attachement path linked to the theme, you will use:

• get_stylesheet_directory() for a child theme
• get_template_directory() for a parent theme (Website without a child theme)

The email Id customer_completed_order is correct to target Customer "completed" email notification.

As you are not using the $order variable argument in your code, ! is_a( $order, 'WC_Order' ) is not needed, so the working code will be:

add_filter( 'woocommerce_email_attachments', 'attach_pdf_file_to_customer_completed_email', 10, 3);function attach_pdf_file_to_customer_completed_email( $attachments, $email_id, $order ) {    if( isset( $email_id ) && $email_id === 'customer_completed_order' ){        $attachments[] = get_stylesheet_directory() . '/Q-0319B.pdf'; // Child theme    }    return $attachments;}

Code goes in functions.php file of your active child theme (or active theme). Tested and work.

For a parent theme replace:

$attachments[] = get_stylesheet_directory() . '/Q-0319B.pdf'; // Child theme

by the following line:

$attachments[] = get_template_directory() . '/Q-0319B.pdf'; // Parent theme