Piracy

If a raster image version would be ok, one avenue to look into is opening up the network tab in developer tools in Firefox, and browsing through all the pages. You may need to zoom in to ensure high enough resolution images are loaded.

At the end of this, click the cog button and "save all as HAR file".

Then you'll need to write some code to extract and correctly order all the page images then feed that into a pdf generator tool.

I got as far as manually inspecting the HAR file with some tool and noting that page images are indeed there, but my need for the textbooks lapsed so I did no further work.