Deterministic PDF rendering

Non-deterministic PDF metadata and host font substitution are the two environment factors that invalidate baselines. Normalize both before diffing, or rely on image diff (which is metadata-free by construction).

Strip non-deterministic PDF metadata

PDFs include /CreationDate, /ID, sometimes /ModDate. These change per run and break byte diffs. Use qpdf to normalize:

qpdf --linearize \
     --object-streams=disable \
     --replace-stream-data=uncompress \
     --remove-attachments \
     out.pdf normalized.pdf

Alternative: rely on image diff (render + pixel-diff) which is metadata-free by construction.

Font-substitution detection

Missing fonts on the rendering host produce visually-different output. Detect via Poppler stderr:

import subprocess

result = subprocess.run(
    ["pdfinfo", "-list-embedded-fonts", "out.pdf"],
    capture_output=True, text=True,
)
if "Font Substitution" in result.stderr:
    raise RuntimeError("Font substitution detected; baseline invalid")

For CI, install the production font pack via the package manager or check fonts into the repo for deterministic builds.