i want to download ios 8.3 directly to install without internet connection on pc

Do you mean that you want to download and do the update via a different computer, or download the update via a different computer and copy it over to your normal computer to do the update on ?

You can update your phone's iOS version by connecting it to and using another computer, but you will need all of your content in that computer's iTunes library (music, apps, films etc) - when updating via a computer the device is backed up (a backup does not contain any of your store downloads, nor your own music, it contains things like settings and apps contents), the iOS update is installed, and the backup restored back onto it (so for the restore to work fully it needs all of the relevant content in the iTunes library so that it can copy it back).

If you just want to download the update on a different computer and copy it back to your normal computer to do the update then that computer will need to be online - when you install an update via a computer it needs to be able to contact Apple's servers and verify the update file.