Create a directory `/usr/share/xournalapp/plugins/UnruledExport/` and place `plugin.ini` and `main.lua` in it. Then enable the plugin "ExportUnruled" in the Plugin menu and restart. There should be a new option in Plugin menu now for exporting directly as unruled PDF and you can also do it using the shortcut <kbd>Alt</kbd>+<kbd>U</kbd>.
- Doesn't work with the latest stable release of Xournal++ yet, as the `export` api hasn't shipped. You'll need to build from source in order to use it.