The piece(s) of paper to which you refer is merely the legal documentation necessarily filed with any and all required governing bodies to establish a corporation as an entity and not the entity itself. As far as "Who's calling a corporation a person?": the SCOTUS.
Although it's been several days since the original post, on the off-chance you didn't get curious and strike out on your own, for more information see: CORPORATE PERSONHOOD.
Seriously, are you stuck holding it until he comes back to retrieve it? Mjolnir stays wherever Thor puts it by affixing its position relative to all the other mass in the Universe; would Thor placing Mjolnir's handle in your hand effectively convert you to a living weapon-rack?
Fair enough; I'll concede that I'm in error on some points of impeachment process. Frankly, I don't like or much care for politics but it seems to be of waxing importance lately so it's looking like it'll be necessary to do some research..
That's an interesting argument and the 4th Amendment does allow for some measure of protection. However, it does not act as an absolute barrier to subpoena; while it is Trump's prerogative to object to a subpoena, he (or representation) must be able to enunciate before a judge specific, over-riding reason(s) for non-compliance. Thereafter, should the issuing body be able to demonstrate reasonable probable cause (in this case, as the subpoena was issued in the course of a criminal investigation into the Trump Organization's financial actions, reasonable probable cause will most likely be in the form evidentiary affirmation), such objections may be overruled by the judge.