Extend UI – open-source UI kit for modern document apps

Extend UI is an open-source, MIT-licensed kit providing 14 customizable components for building modern document apps, including viewers for PDF, DOCX, and XLSX, plus features like bounding box citations, file upload, and e-signatures. It’s designed for developers who need to quickly integrate document handling and annotation into their applications without licensing restrictions. The project is interesting because it fills a gap for high-quality, fully customizable document UI components that are free to use and modify.