There’s also this book which provides a good introduction and overview and is useful for understanding how the format works (although the PDF reference itself is pretty decent too, as far as specs go): https://www.oreilly.com/library/view/developing-with-pdf/978... (You can find a PDF copy if you look around.) EDIT: There’s also https://www.oreilly.com/library/view/pdf-explained/978144932... which might be even better.
However, be warned that the PDF format can be quite complex and is not exactly for the faint of heart. It’s best to use an established library to generate PDF output, like PDFBox, iText, PDFSharp, PDFKit, etc. Those tend to have their own tutorials.
For emphasis: Do not generate PDFs “by hand”! You risk inadvertently generating PDFs that do not fully conform to the spec, and not noticing it because PDF readers are quite lenient in what they accept. A lot of PDFs in the wild are not standard-conforming in some way or other, because their generators were not carefully written against the spec, but against “whatever Acrobat Reader accepts”. This is the bane of every software on the receiving end that needs to process PDFs.
PDF has a lot of features, but it’s not complicated. If your PDF doesn’t contain a 3D model, that’s just some code that takes up room in the library but probably doesn’t even get paged in. A PDF is just a command stream, and the commands for drawing text are straightforward: https://www.oreilly.com/library/view/developing-with-pdf/978...
As to cross platform software, what about Qt, or GTK, or WxWidgets? There isn’t a single decent Electron app. Even VSCode uses CPU sitting there doing nothing. It’s a freaking text editor, what’s happening in the background?!