Invoices can be downloaded one by one from the invoice list. They can also be downloaded for a particular timeframe under 'Settings > Export data.' Make sure that the 'Include attachments' box is clicked in order to download the PDF versions of all invoices.
There is a short tutorial available here:
In German: https://debitoor.de/tutorial/einstellungen/so-exportierst-du-deine-daten-aus-debitoor