Using teaching materials I found on the internet for my own teaching
Note: I am not a lawyer, and this is not legal advice.  Seek a lawyer's opinion if you need a definitive answer.

Merely finding something on the internet doesn't prove you have the right to use it.  


It might be provided free for certain uses, but with restrictions.
It could even be completely illegal (such as stolen content).
Quite commonly, material will be posted without a clear statement of how you can legally use it.  And under most copyright laws, it is presumed that you do not have the right to copy and redistribute material unless that is explicitly granted.


Check the license, if there is one. Some material might be provided with an explicit copyright license, such as one of the Creative Commons licenses.  If so, you can simply check if reusing the material for academic use is allowed.

If there is no official license, but the website has an informal statement allowing the material to be reused, that's a gray area.  Technically, this probably isn't sufficient to grant you a legal basis to use the material.  However, I would probably be happy to proceed on this basis for a low-risk activity like handing out material to students. 

If there is no license, seek permission.  Contact the author and get permission in writing.

In either case, clearly attribute the material.  If the author of the material isn't clear, add a statement.  A copyright license, such as Creative Commons licenses mentioned above, may also have specific requirements for attribution, which you should follow.