The official PDF Reference is very readable, see maest's link. Just start at the bit about the file structure and data types, then the basics about the commands in the Contents stream, the graphics and text states, then whatever takes your fancy. Later versions added some extra complications such as compression of the xref table and object streams, you don't need them unless you encounter them. Don't delve too deep into the bit about fonts unless you have to, it might bend your brain. A tool like qpdf to deconstruct a PDF to an uncompressed form is very handy.

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

This: https://www.adobe.com/content/dam/acom/en/devnet/pdf/pdfs/pd... or a newer version.