One of my favorite #mathematics papers is a paper by Milnor "On manifolds homeomorphic to the 7-sphere" from 1956.
It's only 7 pages long (8 with bibliography). Milnor showed that there exists a smooth 7-manifold homeomorphic but not diffeomorphic to the 7-sphere. The proof is absolutely beautiful and has influenced much of the development of high dimensional topology to this day.
@existential_wormhole thanks. I thought my link led to a free pdf too. at least I was able to download it at the time.
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Free PDF File: @herid http://homepages.math.uic.edu/~kauffman/Milnor.pdf