USB flash drive

Syncthing or rsync?

if they have minimal capacity for installing/configuring/using software, then sending a USB drive via the postal service should be a strong contender

The easiest I've ever used is https://localsend.org/

Very simple, just open it on both computers, select the file and click the other computer.

Syncthing? Never used it on Windows but they do have a client so it should work. That's the simplest I can think of.

Per rule #3, this seems to be a general home computing question and not centered around self-hosting. Please consider adding details to clarify how this involves self-hosting.

Sneakernet was made for this exact situation.

Syncthing, Resilio Sync, or one of those browser based p2p file sends e.g. https://file.pizza or similar.

If both p2p ends know how to use torrents then creating a simple torrent to share to the other peer would work fine. But that requires slightly more IT competence especially if someone needs to open a port forward (ideally you would make sure you have your own port forwarded so the other party doesn't have to worry about this).

If you're doing this more than once it might be worth setting up a simple server e.g. HFS is a nice open source/free HTTP file server, been a while since I used it but it still seems to be active https://www.rejetto.com/hfs/

What about a torrent? You'll have to encrypt with 7zip or something to keep it secure, but that and qbitorrent will do the trick.

ToffeeShare https://toffeeshare.com/

2p2? P2P?

Syncthing.

I recommend https://wormhole.app for the purpose. Drag, drop, leave the tab open.

Check out QuickDAV. I think it’s exactly what you’re looking for. If you’re going across the internet, you’d have to forward a port from your router. Otherwise, if you’re on the same network, it’s really simple.

If all computers are on local network you can use warpinator.

Can we talk about how utterly absurd it is that there isn’t an obvious answer to this question yet? Feels like we’ve gone backwards from the AIM Direct Connect of old.

It’s too big for email, and likely too big for Dropbox or Gdrive unless you have a paid account with them.

That means you’re going to have to get slightly technical. Find a freeware SFTP program that can spawn a server on the host, and connect to it from the client to download the file.

Good luck!

Pairdrop

Maybe https://snapdrop.net/

file.pizza if this is a one off or rare occurrence. If you're doing this regularly, there are better options, provided the person at the "source" computer is competent. A significant question is whether or not these computers are on the same network. I would recommend running a HTTP server if you don't care about privacy, HTTPS if you do. There's no need to buy an SSL certificate, self-signed is more than adequate for this purpose.

It's more complicated to set up, but the advantage is that when you're done you can send the receiving party a link they can open in any web browser, no hassle.

Sync by resilio. I use it between my computers and nas and between my computer and my vps and my nas and my vps, I also use it between my work and personal phone.

If the machines are on the same network, try LocalSend

For this things i use Simple Web Server

It's just one executable and you have a fully running website with your files ready to download

Open the file in a text editor and email the results

KDE Connect should do the trick