Almost everything on the Internet is protected by copyright. Under section 30.04 of the Copyright Act, instructors can use a work downloaded off of the internet if it's for educational purposes.

Just make sure that:

1. You cite the source (and author if known);
2. There is no technological protection measure (TPM) such as a login, password, or other digital lock to access the work;
3. There is no clearly visible notice that explicitly prohibits educational use; and
4. The work was legally posted to the website either by the owner or with their consent.

It's also a good idea to restrict your use of such works to sharing with students via in-class (including online) lectures or posts to D2L.