DOC: Re-enable the PDF build of documentation at readthedocs?

[oesteban]

I have disabled the PDF builds after merging #3124. It is probably something easy to fix, but is there a strong reason to keep building the PDF output?

[effigies]

A PDF is useful just as a single archivable object.

[oesteban]

But are we keeping such archive? Would we like to start with it if not?

[effigies]

ReadTheDocs keeps old versions... We could also add the PDF to the releases, if we want to duplicate to GitHub.