LibreOffice is a free and open-source office suite that is compatible with other major office suites, and is available on all operating systems. It is an independently-developed version of the OpenOffice.org suite. 

The office suite includes:

https://alternativeto.net/software/libreoffice-writer, a word processor with similar functionality and file support to https://alternativeto.net/software/microsoft-word/ . It has extensive WYSIWYG word processing capabilities, but can also be used as a basic text editor.

https://alternativeto.net/software/libreoffice---calc/ , a spreadsheet program, similar to https://alternativeto.net/software/microsoft-excel/ . It has a number of unique features, including a system which automatically defines series of graphs, based on information available to the user.

https://alternativeto.net/software/libreoffice-impress/ , a presentation program resembling https://alternativeto.net/software/microsoft-powerpoint/ . Presentations can be exported as SWF files, allowing them to be viewed on any computer with Adobe Flash installed.

https://alternativeto.net/software/libreoffice---draw/ , a vector graphics editor and diagramming tool similar to https://alternativeto.net/software/microsoft-visio/ . It provides connectors between shapes, which are available in a range of line styles and facilitate building drawings such as flowcharts. It also includes features similar to desktop publishing software such as https://alternativeto.net/software/scribus/ and https://alternativeto.net/software/microsoft-publisher/ .

https://alternativeto.net/software/libreoffice-math/ , an application designed for creating and editing mathematical formulae. 
https://alternativeto.net/software/libreoffice-base/ , a database management program, similar to https://alternativeto.net/software/microsoft-access/ . 

LibreOffice

Windows

Linux

LibreOffice is a powerful office suite – its clean interface and feature-rich tools help you unleash your creativity and enhance your productivity.

Sumatra PDF is a slim, free, open-source PDF/DjVu/EPUB/XPS/CHM/CBR/CBZ/MOBI/FB2 viewer for Windows.
Why another PDF reader?

Sumatra has a minimalistic design. Simplicity has a higher priority than a lot of features.

Its small and starts up very fast.

Its designed for portable use: its just one file with no external dependencies so you can easily run it from external USB drive.

SumatraPDF

PortableApps.com

Sumatra PDF is a slim, free, open-source PDF/DjVu/EPUB/XPS/CHM/CBR/CBZ/MOBI/FB2 viewer for Windows.Why another PDF reader?

Tree Notes is a notes organizer for Windows that allows you to organize and manage all your notes in a tree structure with icons. Tree Notes is very easy to use and light-weight.

Store and manage unlimited number of notes in a single database file.

Tree Notes

Tree Notes is a notes organizer for Windows that allows you to organize and manage all your notes in a tree structure with icons. Tree Notes is very easy to use and light-weight.

Apps with 'Export to office formats' feature

LibreOffice

SumatraPDF

Tree Notes