The PDF translation feature specifically requires connection to Amazon AWS Translate. All other functions can work offline except for PDF translation. To connect to AWS Translate it is required to signup and provide your AWS connection key in web.config
This feature is the single function that requires external cloud service. It also requires you to pay for AWS Translate separately via your own account.
The cloud version uses our own AWS account for this, details are available at https://pdf.co/security
Amazon AWS Translate is a highly compliant cloud based text translation API and API Server connects to it for translating text. See https://aws.amazon.com/translate/ for more details and free signup