In Pandemic, at the end of your turn can you discard a card even when you have fewer than 7 cards, so that the Archivist can pick up that card on his next turn?
No, discarding is not one of your choices of actions, and the discarding to hand limit is very specifically only for when you have too many cards.
If you want to discard a specific card like this, your best available option is to use it for an action: fly to that city, fly from that city to anywhere, or build a research station.
I would beg to differ with Jefromi. As per the On The Brink expansion instructions, if a player wants to discard a card voluntarily it will cost them an action. This is explained on the back page under the Archivist section.