The term "public domain" involves works whose copyright has expired. For instance, Lucy Maud Montgomery's classic novel Anne of Green Gables was protected by copyright when it was first published in 1908, but that copyright has expired because Montgomery died in 1942, more than 70 years ago. This means that Montgomery's novel, as originally published, can now be reprinted or adapted by anyone, without restriction or permission, because it is now in the public domain.
That said, some editions of Anne of Green Gables that are available today contain new bonus content (including introductions, notes, glossaries, and background materials) that is protected by a separate copyright that will expire 70 years after the content creator's death, as per Canada's current copyright term length. But, even when an edition of Anne of Green Gables contains such added content, Montgomery's own text is in the public domain.
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, even though you may still be able to use it for educational purposes because many uses will be covered by fair dealing or the exception for educational use of material publicly available through the Internet.
Note that some copyright owners make clear declarations that certain uses of their copyrighted work may be made without permission or payment. The Reproduction of Federal Law Order, for example, permits anyone to reproduce Canadian laws and decisions of federally constituted courts and administrative tribunals in Canada without needing permission.
