[ogurechny]

You are not reading a PDF document, you are reading a visual representation constructed by a program which is made by people who tear their hair out.

PDF “specification” is not a specification, it only documents the happy path. It never states that behavior of Acrobat remains the holy truth, but in practice undocumented bug-for-bug compatibility is assumed. (We're talking about most basic, universally supported features here.) If ISO was worth their salt, they would at least try to codify the de facto behavior instead of stamping their name on some Adobe-provided document, then it would be horrible but fixed format. A collection of tests would be nice to have, too.

Of course, this “history” is just a promotional leaflet, which describes the “layman approach” they tried to construct. It's a fault not to mention that PDF was, and still is, a foundation of digital print industry, where big vendors solve compatibility problems for mere mortals, and therefore create unwritten rules of what should and shouldn't work.

It is also ironic that they praise the Web, but have to use Web Archive to link to the article from the ancient year of… 2020.