ShiroKuro My impression is similar, that converting from PDF to MusicXML (or any equivalent digitized, quantified format) is still unreliable - and that's where the true technological hurdle lies. That last time I researched this, I could not find a conclusive answer that there is one app that everyone says it works well for this. The most promising seems to be SoundSlice. Other apps include: PhotoScore, SmartScore, PlayScore/Playscore2, ScanScore.
You'd think with today's technology recognizing printed score should be easy. It should certainly be easier than handwriting recognition, right?
Since I use a digital piano, my output to the computer is already digital in the form of MIDI. Every note's pitch, duration, and velocity are numeric values. Musescore has a feature to convert MIDI to written score. And I thought, nice, I can just play a piece, and Musescore will transcribe it. I tried, and what a mess! First problem is Musescore has no idea of time signature, and guessed it badly. Then, it also fails at determining which note is the on the first beat thus messed up on where to put the bar line. It also have trouble separating voices between staffs. And forget about articulation markings. I was really disappointed. And this is even starting from digital source data. PDF scans are essentially images which could present a different set challenge.