I think any document that I e-mail out is a public document, and could easily be forwarded. (Look at all of us quoting one-liners from performance reports in other threads). Ultimately, yes, it would be nice to think that which is report is considered confidential, and kept in house - but the reality is once you send it out, you have no control over the information.
I think it is important to check with your production manager on how the organization views the document. At my organization, we have a fairly large distribution list for both rehearsal/production reports and performance reports. I have to be very careful in what information I put in the report.
If there is confidential information - I will note - there is a HR issue to be discussed, but then keep that information out. With the new medical laws, I have been very conscious to note things like "personal health concern" or "work health concern" in the report, rather then go into details of someone's medical condition.
Given my position at the theatre, I do remind the staff of the theatre that the information in the report is considered in house and confidential. (For example, I had someone from marketing joke with an actor about a missed entrance that was noted in the report . . . bad form, bad form.)
If you have major concerns, putting something in the header of the report - this information considered confidential, and is for internal use only maybe a nice reminder to those reading, this is not to be made public.