Question: If Doc doesn't want to know what's going to happen in his/the future, then why did he tape the letter Marty gave him in 1955, which he tore up, back together instead of throwing it away? Also why did he have it on him at the end to give to Marty, if he had no idea Marty would show up at Lone Pine Mall after coming back to 1985?
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Chosen answer: It's not stated directly in the movie, but it's easy to infer that Doc Brown was subject to the one thing that just about all scientists fall victim to... Curiosity. You can guess that after Marty vanished back to the future, Doc became too curious to resist and assembled the paper back together to read it. In turn, this would give him the information he needed to save himself with a bulletproof jacket, and know the important of why Marty would go back in time when it is 85. And he would have kept the note on him because he DID know Marty would come back to that point. After all, he did send him back off 30 years prior, with the intent of going back to that very day. Doc is intelligent, and would have expected things to turn out that way given the unwritten laws of time travel and paradox.
Quantom X ★
But after this, it poses a new question. Wouldn't he have found a new power source instead so he won't be shot?
No, he is very careful about changing anything in the timeline. So after reading the letter he came up with the bulletproof vest so he could survive without changing anything about the events that led to Marty being sent back to 1955 in exactly the same way. That way he prevented a paradox.
lionhead