PDF - Portable Document Format
PDF, a proprietary format, controlled by Adobe, is a file format used to represent documents in a manner independent of application software, hardware, and operating systems. It was developed in the early 1990's to view document as an electronic image that can be printed or forwarded to someone else.
Today PDF file are used for documents such as magazine articles, product brochures, or flyers by many big websites. To view these documents, you need to install the software - PDF Reader.