A summary report about my company’s 401k plan includes the phrase “You also have the (…) right to examine the annual report at the main office of the plan” and then the address listed is a P.O. Box.
How the f*** does that work?!?! I can only imagine something like the scene in MiB where the door (to the P.O box) opens and it’s a tiny office staffed by inch-tall aliens.
Note, this comes after instructions on how to receive a full copy of the report, and who to contact if you have questions.
You contact them and find out the street address (as well as the hours they are open…).
This suggests a course of action. 
The P.O. box is obviously not the main office of the plan.
I’ll bet that is some statutory language dating back to the creation of 401(k) plans in the 1970s. There are probably required by law to include this exact language.
Nowadays they might send the annual report to everyone without asking, and if not they will probably email it to you on request or make it available on their web site.