A digital signature system includes an Electronic Data Interchange (EDI) server and a portable terminal communicating with the EDI server. The portable terminal includes a touch-sensitive display, and a processing unit includes a receiving module configured for receiving a file from the EDI server, a signature mode control module configured for providing a signature file mode for users to select, a signature file creating module configured for recording signature input through the touch-sensitive display and creating a signature file to record the input signature if the signature file mode is selected, and a file combining module configured for combining the signature file and the file from the EDI server into a composite signature file. A related method is also provided.