Okular - Okular is a universal document viewer based developed by KDE.
LogicLike - Improve logical thinking through engaging brain games
Sumatra PDF - Sumatra PDF is a slim PDF/DjVu/EPUB/XPS/CHM/CBR/CBZ/MOBI viewer for Windows.
Dualnback.works - Dual N-back is a specific brain training exercise, that's been shown by research - it might improve fluid intelligence.
Foxit Reader - Foxit Reader is a free and light-weight multi-platform PDF document viewer.
JobFlare - Brain games for job seekers