Storing and reading documents on mobile devices became common in the last years. Among these, there may also be public documents, but search engines allowing access to such locally stored public resources are still under development. This book presents the theoretical foundations for a search system designed to find similar documents in a network of mobile devices. Document similarity is measured in terms of topic similarity, instead of the common keyword based approaches. The proposed solution takes the special environment of mobile devices into account: it requires few processing power and maintains low communication traffic. This allows smartphones and tablets to keep searching in background without depleting their batteries in a few hours. The book presents several theoretical and experimental results which validate the applicability of the proposed techniques for searching similar documents between mobile devices. The book is intended for everyone interested in document topic identification, very compact topic representation, and classification.