Phone calls are better than files, generally speaking, and you should be calling from a burner; i.e. a pre-paid phone that is not in your name. You shouldn't even give your real name to the reporter on first contact. Reporters take notes and some of them have to share their sourcing with editors. So be really clear with them how they will treat your real identity, if you choose to share it.
Face to face meetings are sometimes better than phone calls. You should assume, when you're handling highly sensitive information, that the reporter's devices may eventually be hacked, bugged or subpoena'd, so make sure that an electronic trail does not lead back to you.
You should carefully choose the journalists you leak to. The best choice will have to be well sourced. That's because the information you leak to them, in most cases, will have to be confirmed. That is, they will have to call other insiders they know and ask "Is X true?" If they don't have other sources, the information you provide will probably not make it to the public.
Reporters also get contacted by a lot of nut jobs, so early on, do what you must to establish credibility. Trust has to be established both ways.