Down load form as PDF - default file extension

If I change the name of a form through the Download Form as PDF process with Collaborator access, I am unable work out how to select a default file extension of .PDF. It means that I have to remember to add the .PDF to my file name which is a pain, otherwise it doesn't save with an extension.

As a Collaborator, I want to be able to save a form with a unique name and it to default as a PDF file. Is this possible?

Author defaults to .PDF