Stirling PDF is a self-hosted “PDF Swiss-army knife” that got massive traction (≈74k GitHub stars) because it replaces a bunch of sketchy online PDF sites with something you run privately. Use it when you need quick merge/split/reorder, convert, redact, OCR, and batch workflows, plus there’s an API for automation. It’s perfect to “see in action” because the UI value is immediate: upload a messy PDF, fix it in 30 seconds, done.