Thats a good point. When I started the pdfs with pictures were too slow to page through and the files were big, and many of the early pdfs were not searchable. But these days there is not so much problem on these fronts. I have both versions on my computer but I still usually use the txt since they are still a bit faster to load and search.
BTW the general resources page at
lists historical books online toward the end … that list is not very complete but still better than nothing.