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
$ diff-pdf --output-diff=diff.pdf a.pdf b.pdf
Another option is to compare the two files visually in a simple GUI, using
$ diff-pdf --view a.pdf b.pdf
This opens a window that lets you view the files' pages, identify their differences and zoom in on the details:
diff-pdf is released under the GPL licence.