Add attachment to WooCommerce custom email

functions.php

<?php

// Path file = yoursite.com/wp-content/file.pdf

function action_woocommerce_order_status_changed( $order_id, $old_status, $new_status, $order ) {   
    // Compare
    if ( $new_status == 'completed'  && $order->get_payment_method() == 'dobirka' ) {
        // Mailer
        $mailer = WC()->mailer();
        
        // To, subject, message
        $to = $order->get_billing_email();
        $subject = __( 'My subject', 'woocommerce' );
        $message_body = __( 'My message', 'woocommerce' );

        // Message head and message body.
        $message = $mailer->wrap_message( sprintf( __( 'Message here %s', 'woocommerce' ), $order->get_billing_first_name() ), $message_body );
        
        // Headers
        $headers = 'Content-Type: text/html\r\n';

        // Path to file
        $attachments = array( WP_CONTENT_DIR . '/file.pdf' );

        // Send an email
        $mailer->send( $to, $subject, $message, $headers, $attachments );
    }
}
add_action( 'woocommerce_order_status_changed', 'action_woocommerce_order_status_changed', 10, 4 );