Here is the use scenario:
Users can access an app, log in, and navigate through routes. The routes are meant to supply users with information, depending on choices they take. At various nodes, there are forms that the user can fill out, which will then populate the fields of printable PDFs. There is a paywall and users can use Paypal to receive their credentials.
I hope to use Node, Express, and React. Not necessarily React if it’s overkill.
Firstly, welcome to the forums.
While we are primarily here to help people with their Free Code Camp progress, we are open to people on other paths, too. Some of what you are asking is pretty trivial in the Free Code Camp context, so you might find that if you’re not getting the instruction and material you need in your current studies, the FCC curriculum will really help you get started. At a modest guess I’d say investing a 4-5 hours working through the curriculum here will really pay off. You can find the curriculum at https://www.freecodecamp.org/learn.
With your current questions, we don’t have enough context to know what you already know or don’t know, so it is impossible to guide you without just telling you the answer (which we won’t do).
It is pretty typical on here for people to share a codepen / repl.it / jsfiddle example of what they have tried so that anyone helping has more of an idea of what help is actually helpful.
Please provide some example of what you’ve tried and I’m sure you’ll get more help.
Happy coding 
Thanks for the response. Here is what I’ve done so far at FCC:
Responsive Web Design
JS Algorithms
Currently working on the API Projects
I’ve also done Udemy courses on React and NodeJS/Express/REST
Unfortunately, I don’t have a link because I haven’t started this project yet. I wanted to get some opinions so I could plan it–as in the past, I usually just start and end up coding myself into a dead end.
Basically, what I’m looking for is suggestions on what tech stack to use for the project. The two parts I’m most worried about is filling out empty fields in PDFs–so far I haven’t found anything that does that, and hooking up an app to Paypal–I’ve done that using plugins at WordPress, but never in an app of my own choosing.
Thanks!