Foxit PDF Editor/Foxit PDF 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.
Right-click on Windows task bar to open Task Manager, then go to Processes > right-click on Windows Explorer to End task, then go to File > Run new task > input explorer.exe to restart Windows File Explorer.