Do you want to save the gantt chart as a PDF file to print it or simply to get an overview of the project/tasks etc. on one page?
Then this feature is what you are looking for.
Simply click on the
... button that appears when you hover over the respective planning object. Every child planning object of the selected one will be included in the PDF. Press 'Download as PDF' and a new dialog will appear. Choose your desired format, press OK et voilá.