By default there is a form definition name added as a display name in Forms Register. This will also reflect in PDF export when you do this. So the Table Of Content (ToC) won't look too meaningful:
To change that you can define a "Display Name Property" per form definition in Forms Designer (when editing the form definition) and opening the Form Settings menu:
Typically you would have an attribute which uniquely identifies the form, i.e. "Meeting Number" for Meeting Minutes form, and you would select this attribute as Display Name Property. Then, Forms Register and subsequently PDF would have a great looking, referable grid list / Table Of Content: