Her whole diary was photocopied, and put into the investigation file.
The investigation file is not public, and it is not even only one, not in one place - there are things deposited at the investigating judge's office, and things which are elsewhere. I think the diary was deposited with the investigating judge.
But even if it's not public, there is still an alarming number of people who have a right to access it. All the defendants attorneys, first of all. This includes attorneys of Sollecito, Lumumba and Guede. And also civil parties plaintiffs, which means Maresca, Fabiani, Perna, possibly also Tattanelli's attorney (if she had one already).
Theoretically any of these people could leak anything.
Nice passive voice.
And then, how does anyone on the list you mentioned find out that the file exists? Does the investigating judge have to send out notices to each of those attorneys every time something appears in the file s/he keeps?