Ranked in these QuestionsQuestion Ranking
Pro Integrated file library
You can add directories to the sidebar. These directories can be from cloud services (like Dropbox for example).
The app can be themed and the layout can be changed around.
Pro Advanced Markdown support
Smartdown supports Critic Markup and MultiMarkdown flavors of Markdown that add advanced functionality, like tables, footnotes, etc.
Con Not Free
Smartdown costs $19.99.