[Const-me]

> I didn't see an obvious open-source tool that lets you dig into PDF internals

That’s a matter of the toolset. I program C#, and I have good experience with that open source library: https://www.nuget.org/packages/iTextSharp-LGPL/ It’s a decade old by now, but PDF ain’t exactly a new format. That library is not terribly bad for many practical use cases. Particularly good when you only need to create the documents as opposed to editing them, because for that use case you’d want to use an old version of the format anyway, for optimal compatibility.