People access and share information over the web and in other digital environments, including digital libraries, in the form of documents such as books, articles, technical reports, etc. These documents are in a variety of formats, of which the Portable Document Format (PDF) is most widely used because of its emphasis on preserving the layout of the original material. The retrieval of relevant material from these derivative documents is challenging for information retrieval (IR) because the rich semantic structure of these documents is lost. The retrieval of important units such as images, figures, algorithms, mathematical formulas, and tables becomes a challenge. Among these elements, tables are particularly important because they can add value to the resource description, discovery, and accessibility of documents not only on the web but also in libraries if they are made retrievable and presentable to readers. Sighted users comprehend tables for sensemaking using visual cues, but blind and visually impaired users must rely on assistive technologies, including text-to-speech and screen readers, to comprehend tables. However, these technologies do not pay sufficient attention to tables in order to effectively present tables to visually impaired individuals. Therefore, ways must be found to make tables in PDF documents not only retrievable but also comprehensible. Before developing such solutions, it is necessary to review the available assistive technologies, tools, and frameworks for their capabilities, strengths, and limitations from the comprehension perspective of blind and visually impaired people, along with suitable environments like digital libraries. We found no such review article that critically and analytically presents and evaluates these technologies. To fill this gap in the literature, this review paper reports on the current state of the accessibility of PDF documents, digital libraries, assistive technologies, tools, and frameworks that make PDF tables comprehensible and accessible to blind and visually impaired people. The study findings have implications for libraries, information sciences, and information retrieval.
Accessibility of Tables in PDF Documents
