Answer
The .fzip file is a Foxit add-on package format. It can only be recognized by Foxit applications. We cannot double-click to open .fzip file directly, but we can install the .fzip file from within Foxit PDF Editor.
To install
Open Foxit PDF Editor, navigate to Help menu, click on Plug-in Management or Foxit Plug-ins, click on Install Plugin.
Navigate to the location folder you downloaded the FZIP Plug-in to, choose FZIP files from the dropdown menu, select the FZIP file and click Open
Note: The Foxit Plug-Ins option under Help was renamed into "Plug-in Management" since version 2025.2 and version 14.0.