WTF, Now I feel like I am in Twilight Zone. At the URL https://www.inf.puc-rio.br/~roberto/docs/peg.pdf I see different PDF documents depending on which Chrome I use!!! In desktop Chrome, I see a paper by Roberto Ierusalimschy, in Chrome on Android phone I see a COMPLETELY DIFFERENT paper by Bryan Ford on the same topic. I've tried clearing the cache and reloading in both browsers at least three times.
@fuxoft
Another thing that I can think of would be an automatic addition of "www", but damn... I love these riddles.
@LukeAlmighty Capitalizing "Pdf" or "Roberto" or "PEG" leads to invalid URL.