How about endowing a small town library? Establish a sufficient trust fund to keep it going for 200 years, establish the rules on hiring of new employees over time, and write it into their mandate to provide long-term document storage for the parent foundation that endowed it. Then, send them everything you want saved, and it’s their job to keep it intact for as long as possible, until the parent foundation asks for it back.
Send them a bunch of routine documents as cover for the famous ones you want to sell.
Fair enough; assuming time travel in a non-predestined universe, instead of mucking about with trying to find some place to secure the documents undisturbed for centuries, simply use artificial aging methods applying the Arrhenius equation to speed normal chemical degradation processes in the parchment and ink. (The question of how to also accelerate the beta decay of 14C isotopes to obtain the correct isotopic ratio is a matter left to the advanced reader.)
How about a cemetery? If there are graves that we know haven’t been disturbed between the days of Ben Franklin and now, could we arrange for some stuff to be in a coffin until now?
Have you considered handing it over to Western Union with explicit directions that it be delivered to someone matching you description, answering to the name Moriarty, at an exact location and at an exact time?
Getting it back would be a problem, and you’d probably run into legitimate ownership issues. Would the cemetery claim it? Would the descendants of the deceased claim it? You might even have a government claim part of it, under “found treasure” rules.
I mean, you’ve got a time machine, right? Play grave robber, at a place where no one knows anything valuable has been stashed for centuries, and make your impossible getaway, and cash in the secret results of the heist a year before anyone would even think to look for a guy who may or may not have swiped stuff…
Buy a cheap vacuum food preserving machine and seal the docs in those vacuum bags (if there is no electricity available when you collect the docs just nab the docs, travel back to the future, seal them and then go back in time to hide them).
Then bring some heavy-duty plastic garage storage bins with a latching lid (keeps water and bugs out) and put the stuff you want in there.
Now, your only task is to find a place to keep them for a few hundred years. Some ideas have been suggested above.
That was my first thought. But if they’re preserved too well, they’re not going to look like they’re 200 years old when he goes back and digs them up. He’ll have to bring an archivist with him to help make sure they won’t get damaged, but will age.
I feel like the best place is going to be somewhere that this kind of stuff tends to be found. In a wall, under some floorboards, in a box up in an attic. Somewhere that they’ll be slightly exposed to the elements, but won’t get wet or eaten by insects or anything else that will actually damage anything.
Sidenote: Regarding the vacuum pump if you don’t have electricity. I was going to suggest bringing a nitrogen tank and purging the air from the bags but a hand pumped vacuum would work just fine and be a whole lot lighter.
It occurred to me that, since the OP has a time machine, they can test as many methods as they can think of. Grab a pile of books and newspapers or whatever from olden times that are worthless, store them under different conditions and pop back to the future and figure out which worked best.
Then, go back, get the good stuff and you know how to best store them so they have max value and no one questions they are the real deal.
This was my thought too. Though would you expect a 18th century chest to last 250+ years in the dirt without leaking? What are you best bets for hermetically sealing your papers for that length of time using 1700s tech? Resin? Unvulkanized rubber? AFAIK vulkanizing rubber isn’t very hard even with 1700s tech, though you’d have to make sure no one sees you do it and copies you, changing history in untold ways.
I had a interesting idea. Buy (or commission) a big heavy tome of some kind with a very thick binding cover. Nothing super valuable or world changing just interesting enough for a libray to accept it and keep it around. Then carve out out enough space in the cover to store your documents, put them in there and seal it up so no one would ever know.
Then donate the book to Harvard library or some other library you know survives to the present day. Travel back to the present day, get academic credentials and come up with a reason to be interested in your book. Go to Harvard, get the book out in the reading room, extract your papers and seal it up again while no one is looking.
Of course risks a significant criminal prosecution if someone notices what you are doing, which not ideal.
Just bring back a modern Pelican case or whatever with you. Unless you were counting on unearthing the thing in public and using the apparent age of the chest as part of the authentication of the documents.
This was another thing I thought about, if your plan succeeds would anyone believe you if you didn’t do something like this? You have a letter with no provenance that you claim is from Ben Franklin. You could show the paper is from 1700s and the handwriting is consistent, but that is easily fakable (by some definition of “easy”). I don’t think it would be taken seriously.