Even if you have the APK file, you will still have problems installing it because the phone is lacking Android 4.4 and, unfortunately, it's not possible to officially install it. Of course, there are ways, but they're complicated and besides that, the warranty and some of the functionality is compromised. I have checked the Samsung-updates website for this phone (GT-I9300) and the latest version available for installation is Android JellyBean 4.3.
In case Kies will manage to upgrade your Android, it will be Android 4.3 and the SideSync application requires Android 4.4.