The Portable Document Format (PDF) was developed by Adobe as a form of electronic paper. They wanted a way to simplify a Postscript document so that it could be viewed and printed on any computer system.
PDF documents are very easy to create. Basically any word-processing or drawing program can generate PDF documents by using the Print command. If the user doesn't have a tool like CutePDF installed then PDFs can be created by selecting the Postscript print driver (Apple LaserWriter), outputting to a file, loading the file in the free Ghostview program and then exporting it as PDF.
More info on PDF is available at wikipedia:Portable Document Format