Please follow the below steps to get a pdf downloaded after the form is submitted.
 
1. Please click the  3 dots beside  Campaigns and select  File Repository
2. Click on  Add Files button on the top right and upload the PDF file to the file repository
3. Once the file is uploaded, click the  3 dots on the right of the file and select  Copy URL
4. Please go to the form and click on the  Settings tab of the form
5. Under  Main Settings, you will find an option  "Redirect to another page"
6. Select that option and  add the URL of the PDF
7.  Save the changes.
 
Please check the below Video: