There is no official, free BlueGriffon 3.1 manual PDF . The closest is the paid user guide or a self-printed version of the built-in help.
Would you like step-by-step instructions to export the built-in help to PDF from BlueGriffon 3.1? bluegriffon 3.1 manual pdf
BlueGriffon is an open-source WYSIWYG HTML editor (successor to Nvu). Version 3.1 was released around 2018–2019 (based on release history). There is no official, free BlueGriffon 3