Guru - So long as within your Terms of Service, or a Disclaimer; you let the public know that not all of the content is yours, under your copyright, and or your responsibility (as I have previously said). The only way a case could be brought against you, is if you refuse to remove the content, or cite the content, which ever the author requests... (though as Seti said, it would be good to add that you will remove it)
Any documents that openly express within it's self that it is alright to re-distribute the content is subjected to those same terms... The only way a case can be brought against you is if you refuse to take it down or cite the work, which ever the author requests.
Also just as a heads up to you, it would be wise to include withing your Terms of Service and (though best) your Disclaimer; that the content that is being present on your site is for the soul purpose of knowledge, and that it is not your or your website's responsibility for others actions with the use of the knowledge; that the user them selves is responsible for any kind of action they choose to use that knowledge for...
Also if someone else decides to try to say the content in which was uploaded, is theirs you have the legal right to ask that they can prove it by providing some sort of proof that they are the original author (I could be wrong but I am pretty sure you do). In a court case, they would have to have the copyright registered at least 1 month prior to a court date. In which case if they are not able to provide proof that they are the originating author, the case will be thrown out.. In order to provide proof of copyright, there must be some sort of hard copy (keep in mind, hard copies these days do have timestamps, if the time stamp is not older then that of which is on your websites server, they have no case). Also the only thing they can sue you for is damages or loss, unless they have had the registration for that copyright at least 3 months prior to the court date. In which case they can then sue you for lawyer fees as well..
But also a reminder to you, prior to being able to sue you, seeing as you do not hold anything of true value, and your not trying to make some sort of monetary gain off of this (again this would lead back to your ToS/Disclaimer), they are inclined to request that you either remove it, or cite the content... If you refuse to do so, then they may have a case to force you to do so... If it can be proven that you have caused some sort of actual damaging to them, or something of the sort, then they can get you for money...
