Displaying (ACF) Advanced custom fields on the PDF invoice

Hi there and thankyou for this plugin! its great.

I would love to know how to show ACF fields on the PDF invoice template. I have tried using the code;

<?php $wpo_wcpdf->custom_field('year'); ?>

But it does not display the year. Any ideas where to next? Any help greatly appreciated. I did search quite a bit and found nothing, but thats me.

Cheers!