PDF files describe page geometry with boxes, not a single page-size field. This checker uses the CropBox that a viewer displays and compares it with the full MediaBox. The analysis runs locally, so confidential briefs, reports and print files never need to leave your device.
The report groups pages by physical dimensions while keeping portrait and landscape orientation visible. That means a rotated A4 page still belongs to the A4 group, while a true Letter page creates a separate size group.