Hello Gregor,
There is no standard program to do such conversion.You may need to develop it by yourself.
Instead of using program,You can try another approach.
If your spools were created with access method G and device type SAPWIN,you can select multiple spools in SP01 and print them from PDF printer (for example PDF-Xchange).
If the spools were created with device type PDF,you can print them to file printer.See note 576973
about how to create file printer.
Thanks&Regards
Jie Bai