Can an attached document to an entity somehow be attached through the system automatically to other related entities? 

​@CYoung you can try setting up an object connection transformation rule between the two LUs. To display the documents connected to PR in the PO page as well. Here it will only display the documents in PO but the connection actually is on the PR.

Refer documentation for details on setting up object connection transformation. <web page url>/Documentation/en//DefineSystemBasics/AboutObjectConnectionTrans.htm

Thanks for the tip.  The documentation link didn’t work directly so I navigated to page help for “object connection transformation” to locate “about object connection transformation”.  Will investigate   further.