File viewer
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A file viewer is a piece of computer software that displays the data stored in a computer file in a human-friendly form. The file contents are displayed on the screen, or they may be printed. Also, they may be read aloud using speech synthesis.
File viewers do not edit files, still it is common for them to be able to save data in a different file format, or to copy information from the viewed file to clipboard.
File viewers have to have sufficient knowledge about the format of the file to be viewed. Even plain text files are not so simple — file viewers may have to handle different codepages and newline styles.
The simplest file viewers are filters that translate binary files into plain text (one example antiword). However, in this way some information may be lost. Disassemblers also fall in this category.
Another common type of file viewer is picture viewer that can display picture files of various formats. Common features here are thumbnail preview and zooming.
Proprietary viewers
For more complex file formats, file viewers are usually provided by the same companies that make editing software that use these formats (viewers may be distributed free of charge, while editors have to be bought). Especially for some complex file formats 3rd parties developed viewers. For example MS Project files can be displayed without having MS Project installed see Project Viewer (http://www.afinion.com).
A file viewer is a limited-functionality software in a sense that it does not have a capability to create a file, or modify the content of an existing one. Instead, it is used only to display or print the content.
The primary reason behind this missing functionality is due to marketing considerations. For example, a popular software program, Adobe Acrobat, can be used to create a content for most computer platforms, under various operating systems. To ensure that people can access the documents created with Adobe Acrobat the software publisher created a viewer software, the Acrobat Reader, and made it available for free. This viewer software ensures that people can create content that is readable on all supported platform, free of charge, thus making it a more attractive solution.
There are many products that can qualify as a file viewer: Microsoft Word or Microsoft PowerPoint viewer are additional examples. In a sense, a web browser is a type of file viewer, which translates, or renders, the HTML markups. Although HTML is plain text, viewing an HTML file in a browser or in a text editor can produce significantly different result.
Although web browser are arguably the best file viewer, since they support many graphic, multimedia and document formats, they are still lacking the output quality and the performance of today's leading software packages such as Microsoft PowerPoint or Adobe Acrobat, so creating and using alternative publishing systems and accompanying file viewers still make a lot of business sense.
Streaming media
There also types of data that are not intended for static display — they have the time dimension. Viewers for such formats are named players. But the essence is the same — presenting file contents in human-friendly form (i.e. displaying video on the screen as intended or playing sound through loudspeakers). And the same problem of different file formats is present.