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.
Select the field to edit and click the edit icon. In the Field Details panel, type in the default value you wish to use. Alternatively, select a “macro” from the dropdown, and the corresponding value will be inserted into the Default value field. Click Save changes to keep this default value.
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.