Developers and retro-tech hobbyists use Java emulators (like J2ME Loader for Android or KEmulator on PC) to run old apps. These emulators require a JAD file to identify and load the Viber application correctly.
If you're looking for a specific version of a for a specific device model, let me know the phone model or operating system and I can help you find the most compatible archive! Viber-jad-file