I had the same thing happen, but for me it was the "o" that appeared as the Umlaut "ü." I first installed the German spell checker but that didn't change anything. I then turned off the option "Recognize Text" under the "Scanned Document" section of the "Edit PDF" Menu. My guess is that the OCR (optical character recognition) is reading in characters and then interpreting them. Since my document was a poor scan, I can imagine that the mapping of my o, or your above 0 falsely generates the letter ü. Not really a bug, just a function of how the scanning works, and in this case, its limitation. Hope that helps.