WHICH FIELD IS POPULATED AUTOMATICALLY WHEN A RECORD IS SAVED AS A PDF?

We have a function where users print an exact form and then save to PDF in order to have a document they can send externally. I would like to make it so that the title of the form automatically populates the file name field when they press SAVE.I notice that the file name  differs from table to table so I am assuming this is automatically populated from some setting on the table... I just cannot figure out which one. 
IS there a way to manipulate the file name? 
Thanks,
Amanda