knowledge should be free

Are you a student? (What are you studying? Don't dox yourself – be a little vague.) Do you have institutional access? It's worth checking with your library for a digital copy; if it's a core text, it should be available.

Depending on the subject, it's worth looking into previous editions or using the library. Or search for the name of the book + pdf and you might find a copy online.

My approach would be to start with the online library. Or the physical library if nearby/on campus. If that doesn't work, try ickdscl's suggestions. If that doesn't work, search for the book + PDF (I tend to trust e.g. libgen more than random websites found on google but I can't say I've ever had an issue, it's just a precaution). If that doesn't work, see how much the previous edition or a second-hand copy of the current edition is. If that's too expensive, consider the edition before that. If it's still too expensive, I ain't reading it lol; I'll start the process again with a similar-looking textbook.

If you are a student, mention the cost to your tutor; they might have a spare inspection copy of the current or previous edition that they could give/lend you. Or you might partner-up and buy a copy with a friend to share/scan throughout the year.

You can always look for an alternative textbook, too. Reading the primary sources for yourself is always the best option but it is time-consuming and it's a skill that must be developed. Textbooks are good for wider context, an overview, helping develop the relevant academic skills (e.g. spotting the relevant part(s) of the primary sources). They can help things click, or help clarify something confusing. They have their uses. If you can't find the textbook, consider those uses – what would you be trying to get from the source? – and see if you can get the same thing from somewhere else.

A textbook chapter on broken windows theory, for example, will discuss however many ideas and concepts. An article on the same topic will likely introduce some, most, or all of those same ideas and concepts before/while presenting a novel argument. An article in some disciplines will need to 'set the scene' and explain the 'state of the art'. So you can sometimes get the same thing as from a textbook chapter by reading the first half-or-so of an article (depending on the subject/structure). Do that for a few articles in case the author missed anything major, and you should be fine.

Just remember that PDFs aren't necessarily secure. They may be able to send data back to whoever created the document. This includes data in other PDFs and about whether you have opened the file. So if you open a PDF of a bank statement and a PDF of a tampered textbook, you might be in trouble. One solution is to turn off scripts in the settings, which may provide some protection. Another is to be careful not to leave PDFs open. Both options could be imperfect but I've never had a problem (if I have, I've not yet found out).

Edit: Yes, knowledge wants to be and should be free!