Prefer simplicity over testability?
The WebService interface with a CreatePDF method is good like that.
The private GetQuery method is an implementation detail that should not be exposed.

But there's something else you can do to test GetQuery.
You can move that logic outside to a different class,
whose main responsibility will be to build query strings.
Then the method will be public, naturally, and you can implement unit tests for it.

The query string builder class can become a collaborator of WebService:
implementations of WebService can call it from their GetQuery methods to build query strings correctly,
with the comforting thought that the utility class is properly unit tested.