(Sorry for the late response.) Well it depends a lot on the site. Since I focus on books and scholarly articles, the ideal way is to find the URL of the original PDF. The website might show you just individual pages as images, but it might hide the link to the PDF somewhere in the code. Alternatively, you might just obtain all the URLs of the individual page images, put them all into a download manager, and later bundle them all into a new PDF. (When you open the “inspect element” window, you just have to figure out which part of the code is meant to display the pages/images to you.) Sometimes the PDFs and page images can be found in your browser cache, as I mention in the OP. There’s quite some variety among the different sites, but with even the most rudimentary knowledge of web design you should be able to figure out most of them.

If need help with ripping something in particular, DM me and I’ll give it a try.

I never said I follow the law, I’m just wondering what the law says ;)

Honestly much of your reply is confusing me and doesn’t seem to be relevant to my questions. This is what I think is crucial:

Just because a file is cached on your device does not mean you are the legal owner of that content forever.

What does being “the legal owner forever” actually entail, either with regards to a physical book or its scan? And what does that mean regarding what I can legally do with the cached file on my computer?

Came here to post this.

This so much. Years ago I’ve had it push Moon landing denial videos in my recommendations, even though I’ve never watched videos on conspiracies or anything of the sort at the time. The closest I got were pop-sci channels such as Vsauce, Numberphile… It’s just trying to hook you onto the garbage content and garbage ideas because they have good and loyal viewership.