Steps to achieve this -
1. Go to survey >> Analytics >> Under dashboard dropdown, choose response >> Response viewer gets opened.
2. Click on the checkbox above to select all the responses at once.
3. Click on the PDF Export to download the PDF in bulk.
4. Here, what you can do is download the PDF for the first 100 responses, then you can change the number to 101 - 200 range and download the PDF again for those responses and similarly for 201-300, and then you can merge them manually by using any external tool.
Refer screenshot: https://app.screencast.com/s91VHLhAewm2j
Refer screenshot: https://app.screencast.com/oV3Sbwmb3O3L6
Refer to the help file: Response Viewer