After some recent hits from 12.33.151.201 and 12.33.151.202, I've determined that Nvidia is the owner / user of those IP's, probably at least 12.33.151.0/24 (a couple of different sources tell me this, even though a BGP lookup only tells me that AT&T owns the entire 12.0.0.0/8 network).
I'm seeing direct file requests for PDF files from my site, no referrer, with this UA:
Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/91.0.4472.124 Safari/537.36
I'm now blocking that /24. This started May 26. I probably need not remind you of the mania / hysteria over Nvidia and their $30k cpu's being used for AI.