I've found a free PDF of my commercial textbook online. Is this OER and can I list my class as ZTC?
Short answer, "No." If your commercial textbook is still being sold by the publisher or is an older edition available for purchase through Amazon or other book sellers, it is not and Open Educational Resource. It's likely that the PDF that you've found is a pirated copy that was posted illegally and is in violation of copyright.
While it is not illegal to share a link to a website or a link to an individual title hosted on the website, by doing so, you are infringing on the intellectual property rights of the author. These types of websites can also sometimes lead to sketchy downloads, which could impact less tech-savvy students by infecting their devices. It is also very likely that the PDF you've discovered is not accessible and because it was posted illegally, it could disappear at any time.
One possible exception to this PDF discovery is the Internet Archive library, which uses the practice of Controlled Digital Lending. These scanned copies are also not fully accessible and most often can be borrowed by a single user for a short period of time (e.g. 1 hour to 14 days). This type of limitation would not guarantee access to every student in your class so would also not count as a Zero Textbook Cost option.
These questions and answers were written by Andi Adkins Pogue and are licensed under a CC BY-NC 4.0 license.