Okular - Okular is a universal document viewer based developed by KDE.
Evernote - Bring your life's work together in one digital workspace. Evernote is the place to collect inspirational ideas, write meaningful words, and move your important projects forward.
Sumatra PDF - Sumatra PDF is a slim PDF/DjVu/EPUB/XPS/CHM/CBR/CBZ/MOBI viewer for Windows.
Joplin - Joplin is a free, open source note taking and to-do application, which can handle a large number of notes organised into notebooks. The notes are searchable, tagged and modified either from the applications directly or from your own text editor.
Foxit Reader - Foxit Reader is a free and light-weight multi-platform PDF document viewer.
Google Keep - Capture notes, share them with others, and access them from your computer, phone or tablet. Free with a Google account.