In the S08E05 of The Big Bang Theory, Leonard and Howard argue about the timeline of Back to the Future Part II. They talk about how the future Biff would go to a different future once he gives the almanac to 1955 Biff. And then when Sheldon came to explain it, I kinda lost it.
He's right. Sheldon is just going with the alternate future created by "Biff giving almanac to his younger self and younger Biff starting to use it in 1958, and this alternate future doesn't contain Doc and Marty as people using the time machine to go to the future, as Doc would be in prison and Marty stepson of Biff as shown in the second part of the movie itself. Hence, this event, of the altered past created by going of the people(Marty and Doc) to 2015 doesn't exist and thus the younger Biff doesn't have the almanac.
Well, when it comes to time machines, we can paradoxically prove or disprove anything, that's the essence of it, if you come to break it(and steal the fun) into understanding.