The comments by Microthrix are not wholly correct. The djvu file may be downloaded by clicking on the HTTPS link. Some improvement of the legibility is possible through View/Layer/Black and White on the djvu toolbar: tick B&W instead of 'Color'.

The PDF files were very poorly scanned. The right-hand pages are often illegible, particularly on the right side.

The DJVU file is more legible. Unfortunately, it is only possible to view it online. It appears that the owner has made it impossible to download or even print pages from it.

The result is that it is impossible to download a fully legible version of this file. Why? I am trying to use it for dissertation research, and I would like a legible copy to write on. I have printed a copy of the color PDF, which is the least illegible version that can be downloaded. I suppose I could spend hours going through each illegible word using the online DJVU file, but why can't I just download the DJVU file? Why is this prohibited? It's frustrating.