Obsidian
Obsidian is a personal knowledge base and note-taking software application that operates on Markdown files. It allows users to make internal links for notes and then to visualize the connections as a graph.It is designed to help users organize and structure their thoughts and knowledge in a flexible, non-linear way. The software is free for personal use, with commercial licenses available for pay.
~Para-quoting from the Wikipedia page
It has community plugins in order to export the canvas to HTML / PDF and also a Calendar Widget. I couldn't find any proper annotation and comment-plugins yet.
Access through both computer and smartphone
As it says above: it's for free, but if you would like to synchronize your files from i.e. your laptop and your phone. You either pay 10 bucks a month, or in case of Android to PC, you participate in corporate cloud culture and use an app like FolderSync.