DjVu was initially developed by AT&T and is used as an image compression technology that is an open sourced alternative to PDF. It allows for scanned documents, photographs and very high resolution images to be distributed via the internet. There are a whole host of DjVu viewers, browser plugins, encoders and decoders available on both Mac and Windows.
DjVu was initially developed by Yann LeCun, L�on Bottou, Patrick Haffner, and Paul G. Howard at AT&T Labs from 1996 to 2001. It promises smaller files sizes than standard PDF's and is therefore perceived to be superior to PDF's primarily due to it's higher compression ratio. DjVu files are typically seperated into three images - the background and foreground (around 100 dpi) and the mask image which is higher resolution (e.g. 300 dpi). The background and foreground images are compressed using an algorithm named IW44 with the mask image compressed using JB2. Ultimately this results in dramatically reduced file sizes.
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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.