There isn't an easy external application to be honest. For the built in templates, you can't actually directly export them, you have to copy and paste. If you wrote some Suitescript though, as you said, you could upload a file and generate a PDF with that. As @Jon Kears said, VSCode with Freemarker is probably as good as your going to get. Freemarker is pretty obscure, so I think a text editor with the extensions installed is going to get the closest. You might also see if they have a Big Faceless Organization PDF extension as well, because that's also a part of the templating language (though much less used than Freemarker. I also should mention that the version of freemarker is quite a few versions back, so some functions in freemarker might not work.