I don't know if the following is the best or easiest way, but:
It appears the Plots.jl doc sources (
https://github.com/JuliaPlots/PlotDocs.jl) are in MkDocs format, which by itself only allows html output; however, the author of MkDocs has released a Python package (
https://github.com/jgrassler/mkdocs-pandoc) which allows you to convert the mkdocs-style sources into pandoc-style sources - which you can then feed to pandoc, to get PDF, EPUB, etc.