Tap the file and choose Reps when your device asks what to open it with. The subject lands in your library with its cards, and you can file it under whichever class it belongs to.
A .reps file is a single subject exported from someone else's copy of Reps. It carries the cards, their photos and their drawings. It does not carry the sender's review history, so every card arrives new and starts its own schedule against your memory rather than theirs.
The file can reach you any way a file travels: AirDrop, Messages, email attachment, a shared folder. The home screen also has an Import button if you would rather pick the file directly.
.reps is the format Reps uses to hand a subject from one person to another. It exists so classmates, siblings and study partners can share decks they built themselves, with the cards, photos and drawings intact.
Reps has to be installed on the device first. If the file arrived inside a compressed archive, expand it before tapping. If the sender renamed the file and lost the .reps ending, ask them to send it again without renaming it.
More answers on the help page, or write to [email protected].