I remember there being apps like xender, easy share etc. which lets you share files by one person activating hotspot and the other wifi you just have to have both party close to each other and they use no data . But they are all closed source and probably spyware and its too much of a hassle to get others to download a file sharing app from fdroid when you need to share big files so is there any website or web app which is private (completely offline after loading, is foss etc.). It also has to work on android and have good enough speed .