Not totally sure what you are after here - have you created your book in iBook Author and want to distribute it as a PDF?
If you have created the book in iBA, you can go to file>export and choose to export to PDF there. You can then distribute that PDF.
If you want other people to be able to get at that PDF, you'd need to go set up website of your own (or some other third-party) as the iBook store does not ditribute PDFs (as mentioned by Tom). Otherwise its down to emailing around, and that can be a problem when it comes to size – many service providers have size limits on email attachments.