Back in college and grad school I used Adobe Acrobat to read all class materials and write notes/highlight texts. One of the benefits was that it was easy to search for key words. You could search your own notes/comments and you had the option to copy the highlighted text so it was searchable.

The pro version also has OCR features so you can do the same with scanned text. I wouldn't pay for it now considering Adobe'a anti-consumer policies, but you can easily find a bootleg copy.

And if you enjoy reading print outs, you can print the pdfs along with all your comments and highlights.

Edit: Forgot to mention this was all on my laptop since typing and reading is a lot easier oh that than a smartphone