glTF (GL transmission format) is a 3D file format that stores 3D model information in JSON format. The use of JSON minimizes both the size of 3D assets and the runtime processing needed to unpack and use those assets. It was adopted for the efficient transmission and loading of 3D scenes and models by applications.
Portable document format (PDF) is a type of document created by Adobe back in 1990s. The purpose of this file format was to introduce a standard for representation of documents and other reference material in a format that is independent of application software, hardware as well as operating system. PDF files can be opened in Adobe Acrobat Reader/Writer as well in most modern browsers like Chrome, Safari, Firefox via extensions/plug-ins.