Document Gallery File Source

I keep versions of my website. The problem is my website is 40gb backed up. It also is very large in size as a site. I would like to be able to change the document gallery source to my main site so that the previous version site will pull pdf’s from my main site. This will allow me to have many sites pulling The source pdf from the main site.

I’ve looked in the wp database and the document gallery folder but can’t find where it stores data for the gallery id’s.

TIA,
— Jeff