I think I already know where the problem came from. A few days ago, I changed the metadata of some of the pdf files who have "missing authors." But as the change did not register on the Reader Library, I tried converting the pdf file to pdf file once again (I read somewhere in the net that this process works for completely changing a file's metadata). While the metadata was successfully updated, I guess the conversion corrupted most parts of the pdf file.
So I guess I'll have to make do with disorganized pdf files.