Dr.Explain - Dr.Explain software is a help file authoring tool.
Markdown to PDF - Super simple way to get your markdown files to PDF
HelpSmith - HelpSmith is a help authoring tool to create CHM HTML help files, manuals, and PDF documents.
NotePlan - Make plans inside an individual markdown note for every day in your calendar. Use it as a journal for your daily tasks and plan todos in advance. For Mac, iPhone and iPad.
MadCap Flare - Documentation for Any Audience, Language or Format
UnMarkDocs - Unleash the Power of Markdown to Write Better Product Documentation