WPD is a text document file format created by Satellite Systems International for their WordPerfect word processing software in 1980. It is a historically popular word processing file format, which can also be opened by most versions of Microsoft Word. Development of the DOS and Macintosh versions of WordPerfect were stopped in 1997 but the Windows version is still under active development by current owners Corel. WordPerfect was the top selling word processing software during the 1980's and early 1990's, and retains a big fanclub to this day (so much so that it was still the second most popular office suite up to a couple of years ago). WordPerfect has been acquired twice - first by Novell in 1994, and then by Corel in 1996 who continue development of the product and the WPD file format to this day.
WPD documents tend to to primarily contain formatted text. WPD files produced in Corel WordPerfect, the company that have developed the format since 1996, are also capable of storing embedded objects such as tables, graphs and images produced in other applications. WPD files contain streams of text, and store formatting information using tags which can be shown as reveal codes in Corel WordPerfect. These highlight how each section of text or object is formatted at a glance, and are therefore useful when trying to determine layout or formatting issues. Like similar file formats such as DOC, WPD files can also contain mail merge fields, which allow information from an external data source, such as a spreadsheet or database, to be inputted into a template.
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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.