I had a few books like that that were directly on a scummy academic editors website. No pdf or usable files. I’m currently far from home, so I can’t tell you exacly what program i used. But i noticed that every page was downloaded in my temporary files as image data (cached version on page). So i had to manually flip a few pages, download them 1 by 1 and naming them correctly. I’ll look ok my pc to try to find the program that did that when I’m back
It was MZCacheview but the same autor made one for chrome and a general one. But theoware is probable right, a brower extension could also do it!