Calibre is a cross-platform open-source suite of e-book software which supports many file formats and reading devices.
Calibre can be downloaded from https://calibre-ebook.com.

LibreOffice (a fork of OpenOffice.org) is a complete, multiplatform and multilingual office suite including a word processor, a spreadsheet application, a presentation tool and more. It uses the OASIS Open Document Format (ODF). Furthermore, it can import/export many other formats including, of course, Microsoft Office Documents (.doc, .xls, .ppt, etc.). Very useful is also the built-in export-as-PDF tool.
LibreOffice can be downloaded from www.libreoffice.org.

Scribus is a full-featured desktop publishing (DTP) application (i.e. ideal for creating posters, brochures and flyers). Being a page layout application, it is different from word processors in the sense that it is concerned with laying out text, images and drawings with a much higher level of precision and control by the designer, which is not possible and sometimes not desirable with modern word processors.
Scribus can be downloaded from http://scribus.sourceforge.net.

OASIS Open Document Format for Office Applications, is an open format for saving and exchanging electronic documents such as memos, reports, books, spreadsheets, databases, charts, and presentations. This standard was developed by the OASIS industry consortium and based upon the XML format originally created by OpenOffice.org. OpenDocument was approved as an OASIS standard on May 1, 2005.
To add ODF-support to Microsoft Office you need to install an ODF add-in: For Office XP/2003/2007/2010, download the converters from http://odf-converter.sourceforge.net.