It seems the GSM and CDMA IPSW files are swapped. Downloading these original IOS versions from Apple servers is considered the only legitimate source for official stock firmware. Where to find the direct links to the iPad Firmware Files for every released firmware version.
In manual iPad update, we download the latest firmware version of iOS or desired firmware of iOS on your computer manually, and use iTunes update feature to manually update iPad.
If you're not sure which firmware file to download for your iPad, then check the post which will help you identify which firmware file to download based on.