Some workarounds only. I haven't modern Acrobat Pro, so this cannot be considered full answer.
The PDF contains spread wide JPG images. You can extract image files from the PDF with some PDF exploding program. I tried it with PDFExtractor. It produced into one folder the spreads as separate JPGs and numerous 1x1 px PNGs which seemed to have no actual function, so they can be deleted.
Any scriptable photo editor would shift the levels in the same way in all files. Unfortunately you do not use Photoshop where the script can be a recorded action which needs no programming skills.
Fast manual adjustment is possible in Paint.NET which remembers the last edit and offers the same settings automatically- simply open say ten spreads and apply the same levels adjustment to all. This is a sample screenshot from Paint.NET:
You need a way to combine the edited JPGs back to a PDF. A not so clever idea is to place them to an otherwise empty layout and print a PDF. I guess people with programming skills could write something better.
You probably have noticed that the text is selectable in the PDF. Adobe Reader either reads and recognizes the text images or OCR result is included in PDF as "OCR-layer". I tried Affinity Publisher (for first 10 spreads only). It found the texts, too. It shouldn't have OCR, so the text is included.
I changed the text color from transparent to black, deleted the JPG and I had 20 pages with readable and editable texts. The work = total a dozen clicks. Unfortunately the fonts were substituted (I haven't originals), but editing the substitution list is possible. Mostly A.Publisher offered Arial by default.
Here's a couple of sample screenshots from Affinity Publisher:
But is it reliable? It's not. It needs 100% proofreading and edits. I see here and there wrong letters. Not many, but there are errors. See footnote 5 on page 8. There are several times ch replaced by eh. It can be a ligature problem, but that's a guess only.
I tested also freeware LibreOffice. It tried different font replacements, but there seemed to be errors in same places than in Affinity Publisher. Font replacement attempts weren't all especially good, so there's more to fix than in Affinity Publisher.
Because Affinity suite programs all open PDFs quite equally but offer different edits easily, opening in Affinity Photo probably is the nearest (without Acrobat Pro) attempt which gives the wanted easy darkening. There the opened pages needed only this (+save as PDF):