Hello, and welcome to the forum
My guess would be digital pics put together in a pdf or scanners (that usually ship with programs for creating pdfs). It can take a long time, I know! It also depends on the size of the book (and of the pdf).
Or maybe you can run the images through an OCR program, so that you can have a proper text (which can be easier to process by the Reader). I am not an expert in OCR, so I can't suggest you a program that
will work without too many problems.