They way you put it the relation will be quickly over. From the way the question was put, both parties have something to gain if these documents are sent over, but the OP the most. He doesn't want to push his wish for security up to the point where the deal is blown off. Pick your battles carefully, that kind of stuff.
As others have pointed out, once the documents are decrypted, there's no telling what happens to them. There's nobody stopping them from sending a copy directly to the NSA for convenience. But in this case, he trusts the organization to do the right thing. Then a once-off practical security measure is simplest: encrypt the document with a password, tell the password in a separate conversation. Once he gets to the point where he has to send another document every day, something like pgp might be in order. But by that time he is talking to other people besides the secretary and it will be a lot easier.