Looks like PDF isn't available, but a downloadable copy of those docs in Markdown format is available from the Github repository linked in the top right of http://docs.civicrm.org ("Edit in Github").
github.com/civicrm/civicrm-user-guide => Download ZIP
This (zipped) download is in a quite readable format, even in a plain text editor. Which tool you view it in will determine whether some features like inline images are displayed; search for a Markdown editor for your OS if you want support for images / visual formatting.
Bonus: it's a great format to submit docs fixes! Perhaps your first submission will improve on "This book is freely available to read online and download in multiple formats." in
In that linked Github repo, you'll see a
mkdocs.yml file indicating that CiviCRM's docs are built via MkDocs. According to this FAQ answer on readthedocs.org, the Markdown/MkDocs format doesn't provide us with automatic PDF generation (yet) but it does make it easy for people to contribute improvements.