You can use a bash script to do it:

 #!/bin/bash

 cd $(dirname $0)

 # Don't screw us up with spaces!
 IFS=$'\n'

 DIRS=$(find . -type d -name "rules")

 for i in $DIRS; do
 rm $i/*.pdf
 done

This iterates through the directories you find in your `find` command and removes the pdf's under each directory.

**EDIT**: This would fail upon encountering a directory with whitespace in its name. To fix that, add the line `IFS=$'\n'` to the beginning of the file, as shown above.

Of course, this is assuming you don't have newlines in any of your filenames. If you do, the solution will become a lot more complicated.