Not sure how many responses you have but you will need to Download each row as a PDF (right click on the row then select Download Response as PDF Form...). You can then use Adobe Acrobat to merge all the PDFs together (unfortunately you can't merge PDFs from the free Adobe Reader).
In Acrobat XI select File > Create > Combine Files into a Single PDF...
The other option is to export the response table as a PDF. This will export the table and not filled out PDF forms.
In FormsCentral's View Responses Tab select File > Export Response... In the dialog select the File Type to be PDF.
Thank you Gen. I came to the same conclusion myself after fiddling around for an hour