For pdf, I don't think it would be easy or at all possible to generate a single pdf from multiple libraries. Though I doubt there is any demand for that nowdays.
I talked to some users at a conference maybe 15 years ago. The expressed great affection for the pdf version of the docs. I also very much like pdf for other non-boost projects I work on.
As the person who used to generate those PDF's, I can tall you it was a right pain to do, plus there seemed to be very few downloads... that might be a chicken and egg situation though. If it was all automated and run on CI that might make it viable again I guess. And yes, by all means modularize the docs, I don't think there were ever that many libraries in the old "monolithic" build anyway? John.