Open the Research Download Tool or Spreadtrum Upgrade Tool on your PC.

In the context of Nokia feature phones (and many devices running Spreadtrum/Unisoc chipsets), the file extension refers to a firmware package. Unlike Android smartphones that often use .zip or .xml files, Spreadtrum-based feature phones utilize the PAC format.

FirmwareFile.com : Provides the full stock firmware package including drivers and flash tool.