Adding any documents to a form is something historically not possible anywhere in Laserfiche. The method is to save the doc you want to the repository, get the URL to it and then use the URL on the form. You can bake the URL into a customHTML field as a clean looking link. Set the URL to a variable the WF uses Set Business Process Variable activity to send to the forms process.
There are cool things you can do with javaScript and hidden customHTML fields to pop up a 'preview' of your doc. iframe it into that field. Adds a bit more functionality. Only downside is the user has to log into Web Client the first time. Just some ideas for you to see there is more than just adding the URL to the form.