ISO 32000 defines the core of PDF: the object model, the file structure, page description, fonts, annotations, encryption, and digital signatures. Version 1 (2008) codified PDF 1.7 as Adobe had last published it; version 2 (2017), known as PDF 2.0, was the first revision developed entirely within ISO, clarifying ambiguities and deprecating obsolete features.
Around this core sit purpose-built subsets. PDF/A (ISO 19005) strips out anything that could break future rendering, making it the standard for archives and registries. PDF/X (ISO 15930) constrains color and fonts for commercial printing. PDF/E (ISO 24517) supports engineering drawings and 3D models. PDF/UA (ISO 14289) specifies the tagging required for screen readers and other assistive technology. Each subset is a contract: follow the extra rules, and the file behaves predictably in its intended environment.