Sumatra PDF - Sumatra PDF is a slim PDF/DjVu/EPUB/XPS/CHM/CBR/CBZ/MOBI viewer for Windows.
APV - Open Source PDF Viewer for Android based on MuPDF library
calibre - Ebook manager, viewer & converter
Foxit Reader - Foxit Reader is a free and light-weight multi-platform PDF document viewer.
Okular - Okular is a universal document viewer based developed by KDE.
Xodo - Try easy-to-use PDF tools that keep your documents safe. Work with your important files across all platforms and devices with the ultimate all-in-one document solution. Your productivity lives at Xodo!