If a document will not let you fill it out the way you need to, you can take manual control of it. That turns it into a custom document, so you can make freeform changes.
If the document is a webform, click Preview first. Otherwise the Manual Control Options icon (3 vertical dots) is already shown on the document.
Save every other section of the webform first, so that what you have already filled in is mapped to the document before you take manual control.
Click the Manual Control Options icon (3 vertical dots).
The original document task is archived, and a custom document is loaded in its place where you can map any fields you want.
See Uploading Documents & Document Templates for how to map a document.
Editing a document that is already manually controlled
Once a document is under manual control, you cannot go back and edit it the way you would a normal task. To make changes, remove the existing task and then add the form again from the forms library.