Paper is described as 'Distraction-free writing app for your iPhone, iPad and Mac' and is a Note-taking tool in the news & books category. There are more than 100 alternatives to Paper for a variety of platforms, including Windows, Mac, Linux, iPhone and iPad. The best alternative is Joplin. It's not free, so if you're looking for a free alternative, you could try Microsoft OneNote or Evernote. Other great apps like Paper are Typora, Simplenote, Manuskript and Scrivener.
Joplin is a free, open source note taking and to-do application, which can handle a large number of notes organised into notebooks. The notes are searchable, can be copied, tagged and modified either from the applications directly or from your own text editor.
No excuses. No distractions. Just write. ghostwriter is a feature-rich text editor using Markdown syntax which makes it suitable for note-taking, document redaction, simple HTML website creation and much more.
Dendron is an open-source, local-first, markdown-based, note-taking tool built on top of VSCode. It supports all the usual features you would expect like tagging, backlinks, a graph view, split panes, and so forth.