Incorporate Travel Document (Passport/ID) to Trip PDF

Incorporate Travel Document (Passport/ID) to Trip PDF

I'm Trying to incorporate "Travel Document Number" (image 1) of a Traveler in the PDF generated for the Trip. But can't find the way to manipulate the PDF generated in the Trip View (image 2)

Image 1:

Image 2:



Some workarround will be to automatically update a Field in the Trip Form. 
But i can't find where to get the API name of "Travel Document Number" field. 

Any suggested solution?