Just ran a test - no brackets. It looks like the message in your photo is coming from an application that's hooking into Exchange. Yes? No? I think there may be an issue with either how it's being delivered to Exchange or how Exchange is syncing it to the phone. Like extra "<" and "." characters in a MIME or MAPI stream. The other mail clients may recognize that they're invalid and junk them, but the PPC isn't.

Try e-mailing yourself a "regular" PDF and see what happens from a POP account. If the brackets are still there, then there may be a corrupted registry entry or bad application install. If they aren't, I think it's a server side issue.

Last possible thought - do you get your Exchange mail via AS or IMAP?

Little randomness there, but hopefully something there helps.