Most of my use cases have to deal with 10-100 PDF small documents, some – 1000-2000, but I don't want the solution to choke on 10GB of huge PDFs (I was just uploading those to Google Pinpoint). So Go or Rust for backend should be good fit.

By cross-platform frontend I meant web/ios/android/desktop. It's probably only Flutter, but I'm looking for other plugins than Syncfusion's one to try. I know that sounds like overkill for many people (website with search suffice), but I already have cross-platform apps that would benefit from this functionality, and web is a fallback there, not the main option.