As WP Travel Pro plugin provides the feature regarding Itinerary Downloads Option under the Tabs of the single trip page, now you can attach the PDF in the booking email using the filter code.
To attach the PDF to the booking email, make sure you have enabled the Downloads option under Pro Modules by going through Admin Panel > WP Travel > Settings >Advanced > Modules Settings > Pro Modules as shown in the screenshot below:

Then, please add the following given code in the functions.php file of your child theme.
function wp_travel_itinerary_pdf_attachment_booking_email( $args ) {
return true;
}
add_filter( 'wp_travel_email_itinerary_pdf_attachment' , 'wp_travel_itinerary_pdf_attachment_booking_email' );
For detail information regarding how to create a child theme, please refer to our Knowledgebase.
Output:

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