Opening iBooks PDFs with Other Apps on iPhone
Another method, simpler, not involving clouds is as follows:


Touch and hold on book's icon in iBooks, and, from pop-up menu
choose "Print", see figure below:





On the preview page, do inverse pinch (two fingers spread):

Now, from Printer options, lower left corner, touch the square to
send to other applications:

And choose where to send a copy - in this example I used Documents
application