Digital culture requires semiotics to invent models for digital representations of paper-based documents in human communication. The objective of the lecture is to specify a three-level communication model (3LCM). The 3LCM has the content plane (first level) and two expression planes - socio-information (second level) and digital (third level). Specifying the 3LCM presupposes a deep study of relationships between mental units of human knowledge (concepts), forms of sign expressions in paper-based documents (information objects) and digital codes of the concepts and their forms in electronic documents that are digital representations of paper-based documents. In the paper an attempt will be made to describe relationships between concepts, information objects and digital codes, to pose relationships between the content plane and two expression planes, to denote features of a digitally signed electronic document, i.e. an electronic document with a digital signature. - In information society, human interests, jobs, rights, and duties will be often connected with digitally signed electronic documents. A theoretically important point here is the perceptive demarcation between signed electronic documents in digital media and signed paper-based documents in social media. The socio-information expression plane contains sign forms of knowledge representation, which are sensory perceived and comprehended by a person. The digital expression plane consists of digital codes of the concepts and their forms in computers and networks. Humans cannot straightforwardly perceive digital codes in a sensory way. However, humans can see their representations in the socio-information expression plane. Consequently, in 3LCM special attention should be paid to the valid transitions from paper-based documents in social media to electronic documents in digital media and vice versa.