Attach a Product's Manual to Its Order Email

add_action( 'woocommerce_product_options_general_product_data', function () {
    global $product_object;

    woocommerce_wp_text_input( array(
        'id'          => '_wpide_email_attachment',
        'label'       => 'Email attachment',
        'type'        => 'url',
        'placeholder' => 'Media library file URL',
        'description' => 'Attached to the order completed email.',
        'desc_tip'    => true,
        'value'       => $product_object instanceof WC_Product ? $product_object->get_meta( '_wpide_email_attachment' ) : '',
    ) );
} );

add_action( 'woocommerce_admin_process_product_object', function ( $product ) {
    // phpcs:ignore WordPress.Security.NonceVerification -- WooCommerce verifies the product save nonce.
    if ( isset( $_POST['_wpide_email_attachment'] ) ) {
        $product->update_meta_data( '_wpide_email_attachment', esc_url_raw( wp_unslash( $_POST['_wpide_email_attachment'] ) ) );
    }
} );

add_filter( 'woocommerce_email_attachments', function ( $attachments, $email_id, $order ) {
    if ( 'customer_completed_order' !== $email_id || ! $order instanceof WC_Order ) {
        return $attachments;
    }

    foreach ( $order->get_items() as $item ) {
        $url = get_post_meta( $item->get_product_id(), '_wpide_email_attachment', true );
        $id  = $url ? attachment_url_to_postid( $url ) : 0;
        $file = $id ? get_attached_file( $id ) : '';

        if ( $file && file_exists( $file ) && ! in_array( $file, $attachments, true ) ) {
            $attachments[] = $file;
        }
    }

    return $attachments;
}, 10, 3 );