In some cases, your custom PDF Template will contain fields that you want to always fill with a default value. In order to do so, click the customize button, navigate to the PDF page that contains the field you wish to map and click on it. 

In the Field Details panel, type in the default value you wish to use or insert a “macro” from the dropdown. 

Note: The "default value" in the PDF field will only be used if there's no "connection". For example, if the PDF field is connected to a template field and that template field is left blank in the engagement, then the "blank" is what will be used to fill in the PDF.