Use a tool like Rufus (Windows) or dd (Linux/macOS) to write the ISO to a USB flash drive or burn it to a CD/DVD. The ISO is typically around 800 MB compressed.
pfSense officially dropped i386 support in the 2.5.x series. With 2.8.0, the codebase is compiled exclusively for 64-bit instruction sets. You cannot install 2.8.0 on an old Atom N270 or Pentium 4 with 1GB of RAM. pfsensece280releaseamd64isogz better
Better handling of IPv6 and simplified global configurations compared to the aging ISC DHCP. Use a tool like Rufus (Windows) or dd