PDF.js is an open-source JavaScript library developed by Mozilla that allows web browsers to display PDF documents directly. It converts PDF files into HTML5 elements, enabling users to view and interact with PDFs without needing additional software or plugins. This library is widely used in various applications and websites, providing a seamless experience for users who need to read or share PDF content. By leveraging modern web technologies, PDF.js ensures compatibility across different devices and platforms, making it a popular choice for developers looking to integrate PDF viewing capabilities into their projects.