This article explains what SQL Compact is and how GFI Archiver uses it.
Microsoft SQL Compact Edition is a runtime environment. GFI Archiver's search indexes require it. Each index makes use of a SQL Compact database, an SDF file aside of the
GFI MailArchiver 2012 SR2 (build 20121204) installs and requires this runtime as a system-wide installation. It can be found under Programs and Features within Windows' Control Panel. The runtime can exist alongside a typical Microsoft SQL server installation.
On a 64-bit operating system, both runtime versions need to be installed, the 32-bit and the 64-bit version. The GFI Mail Archiver installer does this accordingly. Further information about this, as well as a manual download link, can be found on Microsoft Support.
Any other version of GFI Archiver and GFI MailArchiver (including older and newer versions) do not install it as system-wide but place the runtime's DLL files directly into GFI Archiver installation directory.