Brilliant, mdl_files is the key!
I found the moodledata/filedir/00/bc/40charhash and copied a dummy PDF in its place. Now the URL brings me, "Failed to load PDF document" (Opera). Much better that it was. I need to check why it doesn't deliver my dummy PDF. I hope, Moodle does not check the hash sum!
How do I purge this rogue file?
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