1. Go to Control Panel>All Control Panel Items>Devices and Printers.
In Windows 10,please go to Control Panel->Devices ->Printers&Scanners ->Foxit PDF Editor Printer->Manage ->Printer Properties.
2. Right click on Foxit PDF Editor Printer, select Printing Preferences.
3. Choose Layout, click Custom Page Size.
4. Click Add button to add a new form, highlight an existing form, click Edit to modify it or click Remove to delete it.