Hello Martijn.
Your questionnaires should probaly have an Id on the database. You can store all outcomes of the questionnaire in the database, so that your PDF screen only needs to have the questionnaire Id as input parameter, and can read all information necessary for the PDF from the database.
You can use the ScreenToPDF web block to transform your screen into a PDF, and in that case you only need to redirect to that screen in order to trigger the PDF render and download. You can see an example of how to use that in the sample application, which you can download from the forge website: https://www.outsystems.com/forge/component-overview/5641/ultimate-pdf.