I am using appsmith for the app, with a pgsql db. Then n8n to query pgsql with its JSON functions and finally sending it to apitemplate.io to generate a PDF from the JSON and a template. The PDF then goes into nextcloud. With apitemplate you get 50 executions a month for free, which is enough for my use case.
Hope this helps.
(The use case is an equipment maintenance tool for the SCBAs of our fire brigade, where I need reporting for proving that maintenance was carried out)