As the lesson explains, "lo que" is used to refer to an idea previously mentioned, in this case, the idea of finding documentaries boring. If you decide to use "los que", which is absolutely and grammatically possible, the meaning of the sentence changes and it would refer only to "documentaries" and not to the idea of finding documentaries boring.
I hope this helps.
I am sorry that you could not understand my previous answer...Here are both sentences again with their meanings:
1. Le encantan los documentales de historia, LO QUE (yo) encuentro aburrido = I find [the fact] that she loves history documentaries boring.
2. Le encantan los documentales de historia, LOS QUE (yo) encuentro aburridoS (agreement of the adjective is necessary here) = I find history documentaries boring.
I hope that it is clearer now.
It’s a confusing example. Kind of a weird sentence. The idea that someone loves historic documentaries wouldn’t be boring but maybe surprising or strange. The documentaries themselves are what the speaker most likely finds boring. The idea that her friend loves them would strike her as odd or unusual I would think.
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