My suggestion is that if you don't know the source of the material then simply don't include it on the site. I can't cite any legal clauses or documents because I know nothing of US law. So what's below is just personal perspective based on academia.

If you quote remote sources you need to give attribution to them, no matter how small the quote. Even if you're just making reference to someone else's ideas you need to cite that source. If you want to host complete documents which aren't yours then you'll need to get in touch with the copyright owner to get permission since using complete documents (or even significant portions of them) with only citing the source is not enough.

Hosting your own study notes won't fall under the same category since they're your original works. However your school/college may well have an agreement that you will not make public your notes—usually in an effort to squash the problem of plagiarism and to uphold academic integrity (current and future students could just use your work without doing anything themselves!).

Just to summarise, if you don't know the source then don't make it public. You won't get sued or required to have the entire site shut down. At most someone may send you a (legal) request to remove one or more pieces of work if they can lay claim to that work (at least then you know who to talk to with regards to citing the source!). Talk to someone at school/college with regards to hosting your own study notes publicly, don't be surprised if they ask you not to.