Copyright does not last forever - it does expire. When the term of copyright expires, the work is said to come into the "public domain" and is then available for anyone to use and copy without permission or payment. In Canada, copyright protection generally lasts for the life of the creator plus 70 years past their death (though some specific types of works such as sound recordings may have different terms). However, due to the term recently changing from 50 years to 70 years, all works by creators who died in 1971 or earlier are now in the public domain; works by creators who died in 1972 or later, including those still living now, will be protected for their life plus 70 years.
However, content in which copyright has expired may be republished or re-released with additional material. For example, although Shakespeare's plays are not protected by copyright, many of the published editions of his plays contain added original materials (such as footnotes, prefaces etc.). There is a separate copyright in the added original material, based on the life of its creator, but the underlying text of the original work is still in the public domain.
Works can also be in the public domain because the copyright owner has given copyright in the work to the public. Some copyright owners have made clear declarations that uses of their copyright works may be made without permission or payment. Such a declaration may be found on the work itself or the website where it is found. The Reproduction of Federal Law Order, for example, permits anyone, without charge or request for permission, to reproduce Canadian laws and decisions of federally constituted courts and administrative tribunals in Canada. However, restrictions can be placed on the uses that can be made of works and, in this case, you must be sure to use the material accordingly.
Don’t assume that everything you find on the Internet is in the public domain just because it is publicly available. Most of the material you find online is protected by copyright; however, you may be able to use it for educational purposes because fair dealing or other Copyright Act exceptions cover many uses related to teaching. See the FAQs under Instructors – Using copyright protected material from the Internet for further information about using material found on websites.
For more information about duration of copyright protection in Canada see the Government of Canada’s About Copyright page and the Canadian Public Domain Flowchart by the Copyright Office at the University of Alberta (2020), licensed CC BY.