You can open and export digital ebooks in Apple iBooks file format to PDF document format using the export function in iBooks Author on OS X. You will however loose some functionality of ibooks format.
Books created with iBooks Author can only be viewed on an iPad (iOS) or on a Mac OS X, not on any other platform.
You could export your book to pdf format from this app. That can be viewed on any platform, but various features will be lost.
You could recreate your book in the .epub format. That can also be viewed on any platform, also with some features not possible.