Login is a noun (and adj). Log in is a verb. So the right one to use depends somewhat on the context. It's the "login page" but "you need to log in". But it's not super clear in all cases.
I always catch myself saying 'I got it offline ', but I wonder if it is incorrect to say that and instead to say I got it online. What's your take?
If you're saying e.g., "I got this pdf offline", I would argue your usage is incorrect and that saying, "I downloaded this pdf","got it online", or some variant would be more accurate.
That is, unless you would consider, 'got it offline' to be an idiomatic expression for 'downloaded/retrieved-from-the-internet' in whatever region you're from. ~