It depends on what you would like to show in the PDF.
If the Gantt should show in specific gantt units, you can share the view the project-view module is showing in the report view.
So,
- you switch to the that view for example Work breakdown > Status Compact ,
- define the units as required
- limit the date range if needed
- and/or enable a filter, if required
- export as PDF, by File > Export > Image > Format > PDF Vector Image
NOTE: you can include more views in the PDF file. Just click the magnifying glass and click onto + to add more views...
Best regards, Vicky