UserVoice - UserVoice integrates easy-to-use feedback, helpdesk, and knowledge base management tools in one platform that empowers users to speak and companies to understand.
Okular - Okular is a universal document viewer based developed by KDE.
Freshdesk - Freshdesk is a cloud-based customer support software that lets you support customers through traditional channels like phone and email, social channels like Facebook and Twitter, and your own branded community
Sumatra PDF - Sumatra PDF is a slim PDF/DjVu/EPUB/XPS/CHM/CBR/CBZ/MOBI viewer for Windows.
Zendesk - Zendesk is a beautiful, lightweight help-desk solution.
Foxit Reader - Foxit Reader is a free and light-weight multi-platform PDF document viewer.