The term metadata is getting a workout these days. There is internal PDF metadata that can only be changed by a PDF Editor; Finder extended attributes as metadata (e.g. tags, comments, etc.), and Spotlight metadata gleaned by its indexing process. So, when you save a PDF attachment from Mail, Spotlight metadata is potentially (but not guaranteed) is aware of the sender's name and their email address for that document. A Spotlight query can be used to locate that PDF based on a text string in the known email address:
kind:pdf kMDItemOriginSenderHandle:Vik
Which finds the PDF on my Desktop that I originally sent to someone, and contains my email address as Spotlight metadata. I did a Finder Get Info on that PDF, it did not show my email information on that panel. If I email myself a PDF from another Mail account and save that PDF, there is no Spotlight information about my email address as the sender. Spotlight may only pay attention to emails originating from others.
When you move that PDF into that .dmg volume, I believe that you cloak it from being seen by Spotlight, and the email-related information is not found by Spotlight. I suggest that you copy/paste the sender's email address into the Finder Comment field of the PDF via Get Info and before you relocate it into that .dmg volume.