Ubuntu Geek takes a quick look at Diffpdf, a tool to compare two PDF files

DiffPDF is used to compare two PDF files. By default the comparison is of the text on each pair of pages, but comparing the appearance of pages is also supported (for example, if a diagram is changed or a paragraph reformatted). It is also possible to compare particular pages or page ranges.

  Diffpdf - Compare two PDF files textually or visually