Pluto is power. It has a good side and a bad side. When it’s a square, it’s easier for the bad side of Pluto to show itself.
Pluto dominates or controls the thoughts of Mercury. Mercury represents a medium of transport. So, Pluto could have a strong impact on Mercury’s transport or even commercial activity.
As a result of the relationship, Mercury’s relationship with his/her siblings could undergo a transformation. Or Pluto could be insistent and demanding when it comes to Mercury’s siblings.
Mercury’s opinions could be forced to undergo a revolution of some kind.
Pluto is the big boy and Mercury is the smaller one. It stands to reason that Mercury gets the shitty end of the stick.