The PDF file format by Adobe is highly portable format across computer platforms and can be viewed with Adobe Acrobat Reader and other PDF reader programs. Many e-books and other large documents and forms use PDF format of files, and they look very closely like the original printed pages in real books. They are often used by companies that have loads of existing printed material which they want to publish on the Internet since it is quie easy to convert printed papers into PDF files. Adobe Acrobat Reader is a freely distributed software program, which can be downloaded at http://www.adobe.com/. It is available for a variety of platforms and in several languages. If you have trouble opening a pdf file, make sure that you have the latest version of the Acrobat Reader.
DjVu is a file format for digital documents with high performance value and advanced compression technology. DjVu allows very high resolution images of scanned books, documents, digital documents, and photographs for the distribution on the Internet and on DVD. The best program to run .djvu files is called DJVU reader. DjVu viewers are available for the web browser, the desktop, and PDA devices. You should download the full plugin so you can enable printing service. You can download it from http://djvu.org/
The Rich Text Format is developed by Microsoft. It is a free document file format for cross-platform document interchange. Most word processors are able to read and write RTF documents. Here is a (incomplete) list of word processors that are able to open RTF files: Microsoft Word Word Pad (word processor that comes with Windows) Microsoft Works Claris Works