With the Book format, the white pages are automatically inserted in order for chapters to always start on a right page, or to allow an empty secondary coverage.
The default option in LibreOffice does not include the white pages in a document, whether for a printing or an export at toward a PDF format. In order to include the white pages, it’s necessary to active an option.
In order to access the options, instead of using the shortcut in the toolbar, you have to use the menu File→Export As…→Export as PDF, then click the case “Export automatically inserted blank pages“.
Same as previously, you have to use the menu File – Print… (shortcut ctrl+P), then click the case “Print automatically inserted blank pages” in the section “Pages” of the “LibreOffice writer” tab.