I finally found something what does what I want!
Unfortunately it costs $200... now I have to convince my employer I really need this tool...
Btw: According to this page there are several reasons possible why a pdf-indexer might not be able to retrieve the text. In my case it is probably the fact my pdf files are protected against modification... That is probably the reason why other pdf-indexers couldn't find anything.
But if somebody still have another (freeware) suggestion, please let me know!!!