Compares two PDF files page-by-page and reports whether they differ, either as a simple exit code for scripting or, with `--output-diff`, a new PDF that visually highlights every difference between the two inputs. Also offers a GUI mode for interactive side-by-side comparison with synchronized zoom and page-shifting, useful for spotting translation-only or formatting-only changes between two revisions of the same document. Commonly used in CI pipelines to catch unintended visual regressions.