It depend which template you are using.
For example, the InvoicePlane template is found here :
application/views/invoice_templates/pdf/InvoicePlane.php
<footer>
<?php if ($invoice->invoice_terms) : ?>
<div class="notes">
<b><?php _trans('terms'); ?></b><br/>
<?php echo nl2br(htmlsc($invoice->invoice_terms)); ?>
</div>
<?php endif; ?>
</footer>
It is therefore not using the invoice_terms_id. In fact, none of the PDF templates seem to be.
If you want, add the style to the notes div as above as
<div id="invoice_terms" class="notes">