Rich Text Format (RTF) is a file format that enables you to write a text file, save it on one operating system and then have the ability to open it on another. You cannot add images or objects - it is purely aimed at text. You can create a file using Microsoft Word in Windows, save it as an RTF file (it will have a .rtf file name suffix), and send it to someone who uses WordPerfect 6.0 on any version of Windows and they will be able to open the file and read it. Most people would have seen it when writing a 'wordpad rtf' file.
The RTF Specification uses the ANSI, PC-8, Macintosh, and IBM PC character sets. It defines control words and symbols that serve as common denominator formatting commands. When saving a file in the Rich Text Format, the file is processed by an RTF writer that converts the word processor's markup to the RTF language. When being read, the control words and symbols are processed by an RTF reader that converts the RTF language into formatting for the word processor that will display the document.
PDF is a file format developed by Adobe Systems for representing documents in a manner that is separate from the original operating system, application or hardware from where it was originally created. A PDF file can be any length, contain any number of fonts and images and is designed to enable the creation and transfer of printer-ready output.
Each PDF file encapsulates a complete description of a 2D document (and, with the advent of Acrobat 3D, embedded 3D documents) that includes the text, fonts, images and 2D vector graphics that compose the document. They do not encode information that is specific to the application software, hardware, or operating system used to create or view the document.