New web app and consulting LLC formation out of state
Assuming all your activity takes place online and there is no connection to Pennsylvania you might be able to run the company without the need to register it in PA. Keep in mind that in any case only PA corporations and DBAs require publication, not LLCs.
Generally the question of whether your business has physical presence in the state where you live or not is a grey area. You should consult an attorney and/or a CPA to see if your plan to organize the business as Montana LLC (or any other state for that matter) is better than forming it in Pennsylvania. After all, if you are running your business from your home in PA there could be a few business related expenses you might want to be able to deduct.