App not syncing on iPad & iphone

However, when I scan a document on iphone, it does not appear in the app on ipad.

No, that is not possible. You scan from a device, and the document will be save on that device.

Or you have to save the scan document on iCloud Drive,

Its entirely possible, because it is doing the same thing to me. I recently bought a new iPad Pro and turbo scan shows noting in the iPad app, but everything is still in my iPhone

Do you use the same Apple ID on thhe devices?

If you use the same Apple ID on both devices, it would actually be installed automatically on the 2nd device.

Search for the app on the 2nd device and try to download.