pdfToolbox CLI
The command-line interface of pdfToolbox (provided by pdfToolbox CLI) is ideally suited for this kind of integration. It can easily be called from whatever development platform you use (command line calls are available in JavaScript, PHP, .NET…) and the command line syntax is straightforward and very complete.
On top of that (and very important), pdfToolbox CLI is available for macOS, Windows and virtually all Linux flavors. It’s very easy to install (essentially you only need to unarchive the “installation” package) and has no outside library or database dependencies.