If you go to the "print" option under the "file" menu bar, you'll see a PDF button on the lower left side. If you print a PDF version of the book, you should be able to open that in Adobe Reader, where you have the option to "Comment" and "Share". I don't think this is editable, and when I recently tried it, the PDF version showed a white border around the top and side, which is annoying, and I'm not sure how to get rid of it.
Synneve1: I tried your Print to PDF suggestion, and it worked great. But I got the same white border along the top and the right side.
In my print dialog box there is a setting that allows you to change the Scale. I set my Scale to 102% and it was just enough to get rid of that ugly white border without changing the overall look of the page.
Just thought I would pass along the tip!