There are a few apps that I think fit this use case really well.

Languagetool is a spelling and grammer checker that has a server client model. Libreoffice now has built in languagetool integration, where it can acess a server of your choosing. I make it access the server I run locally, since archlinux packages languagetool.

Another is stirling-pdf. This is a really good pdf manipulation program that people like, that comes as a server with a web interface.