From BIF Guidelines Wiki
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.
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While the Book Creator PDF rendering is disabled, 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.