Add option to generate PDFs alongside HTML builds in parallel

[jdillard]

I thought it would be nice to be able to automatically have a PDF as part of your HTML build without having to do additional build configuration to make that possible. This change builds the PDF in parallel using a subprocess and copies it into the HTML output directory, to make the extension more plug an play.

Summary of changes

• Adds simplepdf_parallel_build config option that, when set to True, spawns a concurrent sphinx-build -b simplepdf subprocess during any html based build (e.g. html, singlehtml, dirhtml)
  • Only the final PDF is copied into the output directory — all intermediate HTML artifacts are discarded
  • Inspired by the subprocess pattern used in NVIDIA/sphinx-llm
• WeasyPrint is now imported only when simplepdf_use_weasyprint_api is used, so HTML-only builds no longer fail on extension load
• Added tests

Usage

# conf.py
extensions = ["sphinx_simplepdf"]
simplepdf_parallel_build = True

Then make html produces both HTML and a PDF in the output directory.

AI Disclosure

I used Claude Code to assist with this PR, but checked it's output and made several modifications along the way.

Testing

Currently running in production