diff-pdf is a tool for visually comparing two PDFs.

It takes two PDF files as arguments. By default, its only output
is its return code, which is 0 if there are no differences and 1
if the two PDFs differ. If given the `--output-diff` option, it
produces a PDF file with visually highlighted differences.

Another option is to compare the two files visually in a simple
GUI, using the `--view` argument.
