Finding a direct link for a "FortiClient VPN 6.4 offline installer" can be tricky because Fortinet typically distributes an (a small stub file) that downloads the full package during execution.
FortiClient VPN 6.4 Offline Installer: A Complete Deployment Guide forticlientvpnonlineinstaller-6.4 offline installer
(MSI or full EXE) for faster deployment, air-gapped environments, or silent installations. Extracting the Offline Installer Finding a direct link for a "FortiClient VPN 6