In this case you looking for a tool which compares two PDF files and at least provide you the answer to the question "are those two files the same?". Based on your use-case this means:
- the contained text is the same on the same page of the PDF
- the contained appearance (layout) is the same
As you might know Adobe Acrobat provides a compare function which is very sufficient (see http://tv.adobe.com/watch/acrobat-tips-and-tricks/comparing-two-pdf-documents/) but requires a commercial license and to integrate this function in your automated test environment isn't simple (from technical and commercial point of view).
Fortunately the tool comparepdf is available as free software. It is very simple to install, integrate and use. It provides different compare modes for the scenarios mentioned above. In addition a rough overview of the kind of difference is provided and can be used to integrate in automated test reports.
Once you have identified a difference you might be interested what and where the difference appearce. Therefore the GUI based DiffPDF tool can be used free of charge. Not as powerful as the Adobe Acrobat compare function but in many scenarios it helps to see whats going wrong without the need to buy a Adobe Acrobat license.