You want to perform document isolation to all PDF document.
1. Login to Web Isolation GUI - Profiles
2. Set PDF file type to "Scan" on "Document Isolation Download Profile" in Download Profile
3. Set PDF file type to "View" on "Primary Profile" in Download Profile
4. Enable default Document Isolation rule (in Policies - <policy you are using>) with everything default in "Document Isolation URL" (just like screenshot below). Remove any URL in there except for - https://docisolation*.prod.fire.glass/api/wopi/downloads* shown below.