Stackexchange is a collection of different question/advice groups, some extremely technical and some speculative fandom. Lazy students sometimes try to get the pros to do their homework for them; they rarely succeed. (Pros can recognize homework-caliber problems.)
While trying to discover the source of an obscure mysql error(*), I looked at the sidebar on stackexchange and saw What could an average modern human achieve in medieval times? Those kinds of stories can be fun, but...
The advice was a crazy mixed bag. Your hero will probably die of dysentery within a month if he doesn’t starve (no immediate skills, foreigner w/out a family). Best bet is probably to get to a monastery where they are required to show some hospitality, and then try to show them something about printing or some such skill.
Quite a few commenters had very odd ideas about witches and heresy and technology, and a surprising number didn’t seem to realize that the natives would be quite skillful with their own technologies. You’re not going to make a better arrow or crossbow, or even a lighter, than they would. Given time and an adequate budget you could work up some improvements, but first you have to earn those.
Even printing. Yes, with a printer you can turn out books or pamphlets cheaper per unit, but the upfront cost is high, and they wouldn’t look as pretty—and the market you’d be starting with wanted good looking books. (Why else would monks spend so much time on fancy illustration?)
And as many noted, modern technology demands modern technological infrastructure too—you’re not going to build a useful steam engine. Toys engines you can make, if you can raise the money for them. Reliable motors, no. Remember the story about the pencil, which no single person knows how to make?
Somebody suggested "healer." Not hardly. In Africa even real doctors, who know how to set fractures and other little things our putative hero doesn’t, tend to get called on only after the local village healer fails.
If you knew how to grind smoothly you could try to make magnifiers—if you knew how to make clear glass. If you had some ham you could have some ham and eggs if you had some eggs. You could do a few interesting and useful things if you could demonstrate your bona fides and persuade them to follow you.
(*) It turned out that a newly minted administrator had created a special job to kill off processes that were abusing the database, and his code mistook the nightly backup dump for a rogue user process.