From GamingWiki
Book Creator is undergoing changes
Due to severe issues with our existing system, the Book Creator will no longer support saving a book as a PDF for a short period of time. We are working hard to replace our system and re-enable PDFs within the Book Creator.
Download single pages
While the Book Creator PDF rendering is being replaced, you can try Download as PDF from the sidebar tools for individual articles.
With the book creator you can create a book containing wiki pages of your choice. You can export the book in different formats (for example PDF or ODF) or order a printed copy.
See the help page about books for more information.