1 – Make sure you open the PDF using Safari (Other browsers will not give you the option to save it on iBooks)
After the PDF loads (This could take as long as 10-15 minutes depending on which book you are downloading and your internet connection speed), tap anywhere on the pdf in order to show the “Open in...” menu, click on the “Open in ‘IBooks’” option.
It may take some seconds for the pdf to open as shown below
After this procedure, the PDF will be available on your iBook Library. The book itself will be available offline but in order to work with the interactive activities or to listen to audio files you will have to be online