"Diplomatic channels" for secure communications frequently involves putting the letter in a diplomatic bag and handing giving it to a diplomat to carry on his or her person to its destination.
It is likely that many missions also use encrypted electronic communications too.
However North Korea doesn't have diplomatic relations with the USA. There is no embassy to organise printing out a document. While it is not impossible, it is hard to imagine that Kim Yong Choi would have been printing out the letter in his hotel room before the meeting.
Instead the letter would likely have been produced in North Korea, with the President's seal attached. It would then have been placed in a diplomatic bag and carried by Kim Yong Choi in person to his meeting with the Secretary of State. There was clearly an element of showmanship in the style and size of the letter.
No doubt that the letter was vetted by the secret service before reaching the President's desk.