In this paper, we present the concept of Passive Distributed Indexing, a general-purpose distributed search service for mobile file sharing applications, which is based on peer-to-peer technology. The service enables resource-effective searching for files distributed across mobile devices based on simple queries. Building blocks of PDI constitute local broadcast transmission of query- and response messages, together with caching of query results at every device participating in PDI. Based on these building blocks, the need for flooding the entire network with query messages can be eliminated for most application. In extensive simulation studies, we demonstrate the performance of PDI. Because the requirements of a typical mobile file sharing application are not known — or even do not exist at all — we study the performance of PDI for different system environments and application requirements. We show that due to the flexible design PDI can be employed for several kinds of applications.