reading here: http://symfony.com/doc/current/cookbook/form/form_customization.html
if the name of your field is lets say product[name] you can overwrite the label block just for the individual field:
{% block _product_name_label %}
<label>{{ label|raw }}</label>
{% endblock %}
or for example:
{% block _product_name_label %}
<label>Accept the <a href="url_to_pdf">terms</a></label>
{% endblock %}
just put the code in the template where you render the form and add
{% form_theme form _self %}
so the rendering engine will search for overwritten blocks in the same file first
you can find the default template file in \vendor\symfony\symfony\src\Symfony\Bridge\Twig\Resources\views\Form\form_div_layout.html.twig if you use full stack framework.