Hiding it away is the (relatively) easy part.
First, the physical manuscript itself. The only real concern insofar as preservation goes is to stash your manuscript someplace cool and dry. A manuscript made with typical 14th century materials will survive just fine if it's kept away from detrimental conditions. No need to do anything special to the usual types of parchment or ink you'd find in use throughout the Middle Ages.
(I'm unclear on whether your time traveller is going to retrieve an existing document, or create a new one themself and leave it behind, but it shouldn't matter either way.)
Second, finding a good spot. There are many places that have been sealed for 700 years. Before your time traveller goes back in time, have them identify some buildings they're confident haven't undergone any renovations or experienced any structural damage since they were built, where they might be able to stash the manuscript. The walls of a church or a crypt might be good candidates.
Now, the hard part.
Finally, you state:
The information on the document is only valuable if it comes from a provably original manuscript, not a copy
It's not really possible to do this in a way that leaves absolutely no doubt. 111
(How would you prove today that a handwritten letter is not a copy of another handwritten letter?)
If you need the manuscript to appear contemporary to the 14th century, then here's some things to keep in mind to increase the chances that future paleographers will determine that the document is original and authentic:
1. Use a contemporary language (e.g., don't write in a dialect no one was speaking there in the 14th century). Follow local spelling conventions, etc.
2. Write using a contemporary script.
3. Be careful not to make any mistakes of any kind that might be interpreted as scribal copying errors.
The experts will be able to tell that the writting was produced centuries earlier, and can estimate the antiquity of the materials used. But there might be questions as to just how long ago it was written. Someone could conceivably argue that the writing was made more recently that the 14th century using 14th century materials. 111