I'd say that to covey the meaning correctly, you either got a file from an online resource or else you downloaded a file for offline access. "Got a file offline" as an isolated expression to me sounds like you deleted an online resource. For example: I put a few racy pictures of myself online, but now that I think things through, it'd be best if I got them offline.

Causally, perhaps go with that you got it off the internet.

I'd say that to convey the meaning correctly, you either got a file from an online resource or else you downloaded a file for offline access. "Got a file offline" as an isolated expression to me sounds like you deleted an online resource. For example: I put a few racy pictures of myself online, but now that I think things through, it'd be best if I got them offline.

Casually, perhaps go with that you got it off the internet.