Plagiarism has become a serious problem mainly because of the electronically available documents. An online document retrieval is weighty part of a modern anti-plagiarism tool. This paper describes an architecture and concepts of a real-world document retrieval system, which is a part of a general anti-plagiarism software. A similar system was developed as a part of nationwide plagiarism solution at Masaryk University. The design can be adapted into many situations. Provided recommendation stem from experience of the system operation for several years. The proper usage of such systems contributes to gradual improvement of the quality of student theses.