I Want to Build a PDF Annotate App?

I want to build a PDF annotate app, where users can add things over pdfs. Is node.js a good fit?

Look if you are only thinking about the money from now on and you have no passion to learn web and mobile development, most probably you will not be able to learn the tools you need. To be able to have motivation to continue you should have some passion for it. In term of technology, If you learn Nodejs alone you will not be able to create complete web and mobile apps. You will only be able to create APIs which is basically the back-end (server) logic of your application which is not always necessary to some extent. To be able to create the interface (front-end logic) you need to learn at least HTML, CSS, Javascript first. Then you might need to use Node.js to build the back-end of your application. So i suggest that you become good at HTML, CSS, Javascript. Then learn react native or any JavaScript based mobile development framework. then learn a back-end language. At this stage you can build the web or the mobile application you want.