Hi Markus,
the SAP Help explain exact your problem. I don't get into it. I tried it with release 7.31 and also with 7.02 a few weeks ago and everything worked fine. Just work through the help.
Here is a Wikipage which provides an example-coding:
How to assemble PDF form pages into one file - ABAP Development - SCN Wiki
I also got this link in my pocket,but you sound very familiar to the language
)
Formulare bei der PDF-R�ckgabe b�ndeln (SAP-Bibliothek)
You have to set fp_outputparams-getpdf = 'M', that's a must do, to use the function.
Regards
Florian