Computer Configuration > Administrative Templates > Microsoft Edge
Both
HKLM\Software\Policies\Microsoft\Edge
Determines whether the PDF viewer in Microsoft Edge uses an out-of-process iframe (OOPIF). This will be the new PDF viewer architecture going forward, as it is simpler in design and makes adding new features easier. The current GuestView PDF viewer, which relies on an outdated and overly complex architecture, is being deprecated. When this policy is set to Enabled or not set, Microsoft Edge will use the OOPIF PDF viewer architecture. Once Enabled or not set, the default behavior will be decided by Microsoft Edge. When this policy is set to Disabled, Microsoft Edge will strictly use the existing GuestView PDF viewer. This approach embeds a web page with its own separate frame tree into another web page. This policy will be removed in the future, after the OOPIF PDF viewer feature has fully rolled out.