1. A method, implemented on a machine having at least one processor, storage, and a communication platform connected to a network for archiving a document, comprising the steps of: receiving a document via the communication platform; analyzing, by a feature extractor, the received document in accordance with at least one model to form a feature-based vector characterizing the document; generating, by a semantic extractor, a semantic-based representation of the document based on the feature-based vector, wherein the semantic-based representation has a reduced dimension; constructing, by a reconstruction unit, a reconstructed feature-based vector based on the semantic-based representation of the document, by mapping the semantic-based representation to a feature space of the feature-based vector; comparing, by a discrepancy analyzer, the feature-based vector with the reconstructed feature-based vector to identify a difference between the feature-based vector and the reconstructed feature-based vector; forming a residual feature-based representation of the document based on the difference between the feature-based vector and the reconstructed feature-based vector; generating, by a unified representation construction unit, a unified representation for the document based on the semantic-based representation and the residual feature-based representation; and archiving the document in an information archive based on the unified representation of the document.