Set up this relationship:
Parent::__pkParentID = Documents::_fkParentID
Parent::_fkDocumentID = SelectedDocument::__pkDocumentID
SelectedDocument is an additional Tutorial: What are Table Occurrences? of Documents
Put a button in your portal row with this one line script:
Set Field [ Parent::_fkDocumentID ; Documents::__pkDocumentID ]
Put a container field from SelectedDocument on your layout as the single larger format document for PDF viewing.
Thanks Phil - I was attempting to push a global variable into the FK on the parent table to achieve the same - this has more elegance :)