Hello @FlynnW ,
By default files like PDFs should still open in separate tabs, unless a recipient has setup their browser settings to force the download or has an extension forcing it. That said, when I went through your latest sent email, none of the links that went to PDFs started an automatic download on my end.
Do you know if this issue is occurring only for a live send (aka when the link is replaced with tracking), or is it also occurring in preview / test sends? Is the issue occurring in other browsers or in Incognito / Private browsing?