Or they use them because they have a publishing flow implemented somewhere in the byzantine processes of their company that spits out a nice looking pdf at the end (and maybe a crappy looking 1999 html document)

That's actually been more my experience, yes. :)

People who use PDFs often do it because the content they use it for is of a type (often a series) that has long been produced in PDF, and the reasoning for that in many cases is because in the 1990s people would print it out and read it. In many cases, that's not how people use it now, but PDF is still used because that's the way it has always been done.

I use PDFs for engineering drawings of mechanical parts. It's been done for a long time and is a good fit. There's a specific sheet format and scale, and is meant to be able to be printed easily

...although you need to beware of printouts getting silently scaled. It's really common, when someone asks to print (e.g.) an A4-sized PDF to A4 paper, for the printer driver to rescale it so that the entire document (including margins) fits within the (slightly smaller) printable area of the device. "Shrink to fit [within printable area]" seems to be a common default setting.

(If you're using PDFs for precisely-scaled engineering drawings, I expect you're well aware of this and have a workflow that avoids scaling, but I see people trip over it all too often.)

> I use PDFs for engineering drawings of mechanical parts. It's been done for a long time and is a good fit.

Good for you, but that doesn't change that most of the uses of PDF on the web aren't for applications like that.

Granted, but a surprising number of people (still, in 2020) envision a very static, print-like experience for all web pages. i.e. yes, they "want" a fixed layout but in many cases their reasoning is mis- or uninformed.

They use PDFs because of five monkeys experiment.

This experiment, which apparently never happened [0], just means 'because tradition'.

[0] http://www.throwcase.com/2014/12/21/that-five-monkeys-and-a-...

Serious documents are traditionally published in pdf format, that's why they are published in pdf format.