Add the three custom fields available in WooCommerce PDF Invoices & Packing Slips to your own custom invoice template, see http://wordpress.org/support/topic/adding-text-field-below-the-table.

invoice.php

<!-- Add the first custom fields available in WooCommerce PDF Invoices & Packing Slips to your own custom invoice template, see https://gist.github.com/Willem-Siebe/cf8474b5ca4e72880a04. -->

<?php if ( $wpo_wcpdf->get_extra_1() ): ?>
<div id="extra1">
	<?php $wpo_wcpdf->extra_1(); ?>
</div>
<?php endif; ?>

<!-- Add the second custom fields available in WooCommerce PDF Invoices & Packing Slips to your own custom invoice template, see https://gist.github.com/Willem-Siebe/cf8474b5ca4e72880a04. -->

<?php if ( $wpo_wcpdf->get_extra_2() ): ?>
<div id="extra2">
	<?php $wpo_wcpdf->extra_2(); ?>
</div>
<?php endif; ?>

<!-- Add the third custom fields available in WooCommerce PDF Invoices & Packing Slips to your own custom invoice template, see https://gist.github.com/Willem-Siebe/cf8474b5ca4e72880a04. -->

<?php if ( $wpo_wcpdf->get_extra_3() ): ?>
<div id="extra3">
	<?php $wpo_wcpdf->extra_3(); ?>
</div>
<?php endif; ?>

Breaks website for me. Outputs only HTML without styling. Uncaught error: Call to a member function extra_1() on null in ...

Finally solved it by replacing $wpo_wcpdf->extra_1() with $this->wcpdf->extra_1().

Thanks for your contribution PhilipsEkuma, worked for me!