Stirling PDF is an open-source alternative to commercial PDF tools that you can host on your own server. It bundles dozens of features: merging, splitting, compressing, converting, OCR, adding passwords, removing metadata, and more. The web interface is organized into categories, and there's also an API for automating tasks. Because it runs on your own infrastructure, your documents never leave your control, which matters if you work with confidential material.

It's written in Java and can be deployed with Docker in a few minutes. The project is actively maintained and has a growing community. If you already run a VPS or home server and want a private PDF toolkit, this is one of the better options. It's free and there are no usage limits or watermarks.