PDFs are horrible as eBooks, but in a manual I prefer the fidelity of a PDF than the reflowing of an ePUB, or the fickleness, slowness, and potential unavailability of a web page.

A well structured and correctly indexed PDF is a godsend, because you just open the table of contents, quickly locate what you're looking for, click and there is your answer. As I said, IBM excels at writing documentation. Every language has a Language Reference manual, and a Programmer's Guide manual. The first is for reference, the second is to learn how to use it, including examples.

Don't take my word for it, you can check out, for example, the COBOL manuals here: https://www.ibm.com/docs/en/i/7.1?topic=languages-cobol

I think it depends. For something well-structured like a cookbook I'd prefer PDF in most cases. But it really varies. For example I find most guidebooks do a decent job of Kindle these days and have actually switched over to a significant degree because it's really useful to have everything on my phone.

for this specific use case a single html file that you open in the browser might be the best option (at least php offer that option and I appreciated it).

The first PDF I had was a 'bonus' book that came on a CD included with a programming book. There was a compiler along with sample code from the book, and then something like SAMS Teach Yourself Visual C++ in 21 Days.

I absolutely hated it. My screen was only 640x480, so I could barely see a whole page, much less see my IDE at the same time. These were the days of single 14 or 15 inch monitors, not dual or triple 24" widescreens.

It was also a little on the slow side. Early Pentium, 8MB RAM, 4X CD-ROM...

These days I'm delighted to see a PDF. There's a table of contents and index, they're searchable...copy and paste usually stinks, though.

PDF's are forever. They print nicely. They don't get lost when the project upgrades a major version, you can still use them when there's a DNS issue with your ISP, you can use them when you don't have internet...