Install docbook-xsl-doc-pdf by entering the following commands in the terminal:
sudo apt update sudo apt install docbook-xsl-doc-pdf
stylesheets for processing DocBook XML files (PDF documentation)
DocBook XSL stylesheets are modular XSL stylesheets for processing documents composed with the DocBook XML DTD and its derivatives. . This package contains the documentation for the DocBook XSL stylesheets and explanations for the parameters you can use in PDF format.