That UA is familiar. I don't have a lot of pdfs, but the Googlebot-- and also the Googlebot-Mobile-- will occasionally pick up one that's attached to an e-book.
This particular batch of pdfs is new to the site. They used to live somewhere else, with only a nofollow link pointing to the top one ("frontfile.pdf"). The link is in such an obscure location, it took Google almost two weeks to discover the change. Followed, apparently, by 27 minutes to find and process the links within the pdf ... and then a day and a half to make contact with the pdfbot.
I have never seen that stripped-down Googlebot UA before. Granted, I didn't fine-tooth-comb raw logs going back to the dawn of time. But I looked at some pretty representative chunks. Never anything but the normal Mozilla-clad Googlebot. Or Googlebot-Mobile, which wears even more clothes. Only spoofers use the minimalist version.
I've had that UA enabled for a long time without following how it worked. Trawling through my security logs the UA only hits a few pages of one site - pages that have PDFs.
Now blocked. I also added ^Mediapartners-Google/ ^Googlebot-Image/ to the blocked list. We don't use adsense anywhere, although G does visit to check (surely it should know?!) and we do not (usually) allow image-cutting.