I have updated the documentation build configuration to build a LaTeX-generated PDF in addition to the website every time it refreshes. This can be found at https://raw.githubusercontent.com/OSGeo/proj.4/gh-pages/proj4.pdf and it is linked on the front page of the HTML website. It would be useful to steal LaTeX builder configuration from the MapServer or PDAL projects, which both have quite nice PDFs being generated from Sphinx in a similar approach.
The Sphinx+LaTeX combo can be a little bit finicky. First, SVG isn't supported, so we need to be feeding PNGs or JPEGs to it. If we switch to SVG for the example renderings, we will have to workflow the conversion to PNG as part of the documentation build process for the LaTeX builder. Second, unicode, math expressions, and heading levels can all cause it to blow up in unexpected ways. The documentation builder should error with a marginally helpful error message that you might have to try/catch for a while to figure out how to make happy. You can build the PDF using Docker yourself. See the build_docs.sh script in the ./travis directory for instructions.