Quip - Quip is the modern productivity suite that simplifies your life and helps your team get work done faster.
Okular - Okular is a universal document viewer based developed by KDE.
Google Docs - Create a new document and edit with others at the same time -- from your computer, phone or tablet. Get stuff done with or without an internet connection. Use Docs to edit Word files. Free from Google.
Sumatra PDF - Sumatra PDF is a slim PDF/DjVu/EPUB/XPS/CHM/CBR/CBZ/MOBI viewer for Windows.
Trello - Infinitely flexible. Incredibly easy to use. Great mobile apps. It's free. Trello keeps track of everything, from the big picture to the minute details.
Foxit Reader - Foxit Reader is a free and light-weight multi-platform PDF document viewer.