I always get the error message: "Invalid file extension" when I'm trying to upload a PDF file? Does anyone know how to solve that problem?
When you create the field file, you need to definy the extention. You have already created the field but you can always modify it. Go on your content type, edit the field file adding pdf on "Allowed file extensions *"
You can add all the extensions you want. Separate extensions with a space or comma and do not include the leading dot.
In your content type definition, you must define file extensions. Have you added pdf extension?