The spec is really well written, and will take you far if you just want the basics (i.e. not the scripting stuff they added after 1.4 which is around where you should usually stop if you just care about printing). My only issue with it is when it comes to fonts or images, you'll have to break out to additional specs to understand those formats since PDF is more of a container.

I do like this minimal example as a way to get started and see how a very basic PDF is built.

https://brendanzagaeski.appspot.com/0004.html