The spec is pretty amazing and very readable, for anyone wondering if they should look into it.

My top tip for understanding PDFs is to take one that you have a decompress it then open with a text editor.

    mutool clean -d in.pdf out.pdf

What you'll find is very approachable for a developer. It's a tree of nodes of different types. Some are dictionaries, some are streams of other data. All of them are documented in the spec. There's all sorts of wonderful corners, like spot printing colors.

PDFs are actually ok, wait till you dig into the fonts. Now there's the real dark art of the ancients.