Okular - Okular is a universal document viewer based developed by KDE.
Scrivener - Scrivener is a content-generation tool for composing and structuring documents.
Sumatra PDF - Sumatra PDF is a slim PDF/DjVu/EPUB/XPS/CHM/CBR/CBZ/MOBI viewer for Windows.
Final Draft - Use your creative energy to focus on the content; let Final Draft take care of the style.
Foxit Reader - Foxit Reader is a free and light-weight multi-platform PDF document viewer.
yWriter - Free writing software designed by the author of the Hal Spacejock and Hal Junior series. yWriter6 helps you write a book by organising chapters, scenes, characters and locations in an easy-to-use interface.