이게 CDC 사이트에 나온 공식 답변입니다. 탑승 전에 항공사에 보여줘야 하고, 입국시에도 보여달라 할 수도(potential) 있다고 하니 copy 를 가지고 입국해야 할 듯 하네요.
What kind of documentation of my test result do I need to present?
CDC requires that air passengers arriving in the US have a paper or electronic copy of their test result for review by the airline before you board and for potential review by public health officials after you arrive in the US.
https://www.cdc.gov/coronavirus/2019-ncov/travelers/testing-international-air-travelers.html