Typically Avast is pushing the online installer (avast_free_antivirus_setup_online.exe) which is really small (about 5 MB) but requires Internet connection to complete the installation as the biggest part is downloaded during the installation process. Sometimes you need to get Avast on a computer without the Internet connection and that’s why you need to download the offline installer (avast_free_antivirus_setup_offline.exe) in full size which takes almost 300 MB.
Avast offers standalone installers for all their product from antivirus family (Free Antivirus, Pro Antivirus, Internet Security, Premier) to Cleanup or SecureLine.
Offline installer contains the exact version of Avast as online installer but you can transfer it on a USB disk and install basically anywhere. Even though the computer doesn’t have an Internet access it could get infected (for example when you transferring data from USB sticks, CDs or DVDs) and that’s why you need Avast Antivirus protection. All Avast offline products are fully compatible with Windows 10, 8, 8.1, 7, Vista and XP (32-bit or 64-bit).