Consider using ImageMagick's
composite tool to combine the Excel PDF and your background image:
composite program to overlap one image over another. See Command Line Processing for advice on how to structure your composite command or see below for example usages of the command.
We list a few examples of the composite command here to illustrate its usefulness and ease of use. To get started, lets overlay a smiley face over a rose:
composite -gravity center smile.gif rose: rose-over.png
You will probably need to write a short script to deal with each page, but the result should be more resilient and should work across multiple versions of OS X.
You can use Automator to split out each page of the Excel PDF and, after compositing with the background, another Automator Action can recombine the pages.
Vector Art Applications
There are numerous vector applications available on the Mac that can achieve the result you are looking for:
Working editions of pdftk for OS X 10.9 and 10.10 appear to be available, see How to install pdftk on Mac OS X for build details and various forks.
pdftk on OS X 10.9+
For 10.9 and 10.10, try using the quantiverge edition of
pdftk on homebrew. Comments suggest this works on OS X 10.9 to at least 10.10.4.
Install homebrew and then use the command:
brew install https://raw.github.com/quantiverge/homebrew-binary/pdftk/pdftk.rb