If you're compiling millions of documents, many with thousands of pages, you probably need something very universal. LaTeX is boring tech, off the shelf, ready to use. It might take some work to figure out the initial setup with regards to templating and everything else, but after that, you can be generally pretty content that it will handle most things you throw at it just fine.

I am currently preparing to switch to DITA. The learning curve is steeper at the beginning, but I find the overall concept of topic-oriented, information-typed authoring with content reuse very attractive.

Some people might say that writing in XML is annoying, but it isn't if you have a decent XML editor. In my case, it is Emacs nXML mode. Customisation is possible with DITA-OT [1] and plugins, and yes, it is also based on XSLT. Overall, I think DITA is an industry-proven XML powerhouse. It may be boring, but it has huge potential for anyone with advanced documentation requirements.

[0] https://dita-lang.org/dita/archspec/base/introduction-to-dit...

[1] https://www.dita-ot.org/

I’d suggest LaTeX because of its strengths with tables, mathematical notation, and similar content.

LaTeX notation works well for this and can be easily converted to both web and PDF formats.

But compilation speed definitely needs improvement.

On the other hand, how often do you actually need to compile thousands of pages into a single document? That’s really an edge case.

What else would you use to generate PDFs from a text-based template?