UGH! We get requests all the time at work for information proving the existence of these things.
From my stand point I can truly say that the USAF has nothing on file proving that UFOs are real. During Project Blue Book, all USAF bases were required to log in all UFO sightings and of all of them, only 1% percent could not be explained.
More than likely, the 1% were experimental aircraft, thus being classified at the time, the USAF could not comment.
Just my 2 cents...