I suppose they might want to encrypt a pendrive in case it was stolen from the friends housse. But you could do something similar with a piece of paper.
Pendrives aren't known for storing particularly well over the long term so they probably aren't a great choice anyway.
I suppose an overheated safe would not be good for either a flash storage or paper. Maybe engrave the pdf on a piece of metal :-)
That's what the scientologists did with all of L Ron Hubbard's books.