Foxit PhantomPDF/Foxit Reader has already been set as default PDF program. But PDF Preview in preview pane of Windows explorer is fixed size and small as a thumbnail. You can see what the preview looks like in the following figure.
1.In the Windows File Explorer, open the Folder Options, see whether the option Show preview handlers is checked or not. If it is unchecked, make sure to check it and click on Save button to apply the change.
2.Restart Windows File Explorer and try again.
Why the Windows Explorer PDF preview stops working?