You can find a PHP package to merge multiple RTF documents here :
www.rtftools.net
Here is a short example on how to merge multiple documents together :
include ( 'path/to/RtfMerger.phpclass' ) ;
$merger = new RtfMerger ( 'sample1.rtf', 'sample2.rtf' ) ; // You can specify docs to be merged to the class constructor...
$merger -> Add ( 'sample3.rtf' ) ; // or by using the Add() method
$merger [] = 'sample4.rtf' ; // or by using the array access methods
$merger -> SaveTo ( 'output.rtf' ) ; // Will save files 'sample1' to 'sample4' into 'output.rtf'
This package allows you to handle documents that are bigger than the available memory.