Need some guidance about the tools to create a web application that can produce and let user download document using variables on webpage

I have some experience with c++ using windows application and unity using c# but i dont have clue about web interface backend front end configuration.

I have time so i can take time to learn a newframe work but i am not sure where to start.

What i want to do is basically 1- Users will create accounts 2- For each user there will be options to add new files with some variables, like name and id numbers. 3- Those variables will fill in the gaps in a document and this document can be downloaded or printed out.

I can try this in a non online environment but the program should be online for best efficiency andi really am not sure which frame and language i should use.