get_document_notes()

in invoice.php there is a table cell:

<div class="document-notes">
<?php do_action( 'wpo_wcpdf_before_document_notes', $this->type, $this->order );?>
<?php if ( $this->get_document_notes() ) : ?>
<h3><?php _e( 'Notes', 'woocommerce-pdf-invoices-packing-slips' ); ?></h3>
<?php $this->document_notes(); ?>
<?php endif; ?>
<?php do_action( 'wpo_wcpdf_after_document_notes', $this->type, $this->order ); ?>
</div>

Where can I add content that will be called by $this->get_document_notes()?
Thanks!