KLP Publishing (Scale Modelling Guide Books) is an Australian publisher that specialises in modelling guides of specific topics and in PDF format. Publisher Kevin Futter has a clear vision for this. In short, the publisher does not want to have the limitation of real printed books, including unsaleable stocks,limited circulation against costs, limitation in pages also with regard to the costs. He certainly has a point.
You do not have these limitations with the PDF format, even the resolution of photos can be increased in a PDF so people can use PC or IPad to “blowup” interesting photos ideal for details, especially in modelling,