It is a digital marketing business with clients in the US, but not physical presence or employees or contractors in the US. The LLC is registered in PA. I am a US citizen. Would one qualify for FIFE?
In short No.
There are laws that if you sell to anyone within a particular state in the U.S. you must obey each state's tax laws. However for the majority of the tax laws the exception (loop hole) is in the digital services realm - where you can claim income as 'advisory / non-physical services' and not pay in any of the states except for your own. If you are incorporated in a particular state you do have to pay taxes there - regardless of where you 'conduct business from' you are operating out of PA.
I don't think you have to pay federal (unless your income surpasses their bracket which I think is $100k+ for those residing outside U.S. i.e. military contractors) But you do have to pay PA state income taxes that's why is wise to incorporate any form of business in states with the lowest tax brackets such as Delaware, Wyoming, Nevada...