It means that the document is the same subject you were talking about in the email. It's like similar. E.g.: I'm talking to you about Jonh's criminal life then I'll send an email to you that shows the same (subject about Jonh's criminal life).

When the term "the same" is used, it always requires some context, something that was said before. It means "the same thing that was mentioned in the previous text." You send an e-mail that talks about hydraulics. He sends an e-mail in return that says "This document talks about the same." = This document also talks about hydraulics. Or, this document talks about the same thing that you talked about in your message.

thanks!

As a matter of fact, the rule is, in most cases, you have to add the "the" before "same", "talk about same" would be plain wrong, actually. No matter formal or informal, that's the grammar rule here. Only in a few cases, eg, "same old, same old", and this, is actually called slang.