## 1 Introduction  
Over the past few decades, the number and the variety of industrial products has grown, as well as the complexity of machinery in industrial plants. Furthermore, Industry 4.0--that is the trend towards the computerization of manufacturing--is leading to an exchange of a great quantity of technical information among machines, storage systems, production facilities, and people [1]. A category of information associated with a product or a machine is that of technical documentation, including use and maintenance manuals, operating instructions, and installation manuals [2]. This documentation is composed of technical information assets that may describe a component, provide instructions for a task, data, and so on. Most of the technical information assets are increasingly available in various digital formats. They are usually conveyed through visual cues: video, text, image, CAD models, and so on. Consequently, also the devices used to convey technical documentation are evolving from the traditional ones as paper manuals and monitors, to handheld devices and holographic displays. A key technology that is going to be used for displaying these next-generation manuals is augmented reality (AR). Operators can be different one from the other. They can have different levels of expertise and experience. As to experience level, we can distinguish from novice to expert operators according to the knowledge acquired performing technical procedures. However, a same operator could have a greater expertise on a particular machine/product than on others. Experience can regard different skills as assembling skills, using the tools, problem solving, and so on. Thus, some operators may want to read the explanation of each step, while others may understand everything by just looking at an image of the procedure. Operators could have also different needs when consulting a technical manual given a certain task, someone may want to check a single step, others may have to follow the entire guide step-by-step. Younger people are digital natives [3; 4] and thus more oriented towards innovative displays, whereas older people would be more attached to traditional media. Finally, people speak different languages and come from different cultures [5], then also technical documentation is affected by these differences. The domain of technical documentation covers several application environments. This might allow the use of some specific user interfaces for conveying information while making impossible or dangerous to use some others. For example, some application environments may not require great attention to the real world, while others may be critical or dangerous tasks, allowing attention loss on reality for just a few seconds [6]. As a result, the increasing number of technical documentations to produce, the variety of ways to convey information, the differences among people, and the variable environmental conditions, make sure that the authoring and the management of technical documentation becomes an issue. Many works on adaptive AR interfaces propose algorithms to determine optimal text styles (e.g., text color and drawing style, position of labeling), based on the background of the environment [7; 8; 9]. However, these works do not consider dynamically altering the information content or other contextual elements. The management of large informative systems is a problem which has been broadly investigated in many fields, for example in information retrieval by research engines. Advanced context-aware information retrieval systems have shown excellent results in the e-learning and e-training contexts [10; 11]. However, to our best knowledge, the results from these studies often ignore the 'people' factor. The user must manually search for the information they are looking for, using some research keys. This requires previous knowledge in the domain that they may not have. Moreover, of similar approaches found in the literature [12; 13; 14; 15; 16], most do not specifically consider a large set of context features.