When a PDF is used for both printing and displaying. It will most likely
contain a crop box in order to hide print margins when displaying the PDF.

Use Poppler's parameter to automatically use the crop box (visible box)
rather than the media box (printable box) in order to remove those margins
when drawing the PDF.

See https://manpages.debian.org/testing/poppler-utils/pdftoppm.1.en.html