Some Customer may want to install the Antenna on the Attic so that you don't need to run a cable from outside and avoid the bad weather.

Yes, you can install the Antenna on the attic. But the shielding on the roof may cause the signal strength weaker than put in outside. So you need to check if the signal level is god enough for most of the channel first before you install it on the attic.

You may expect the signal a 10-15% weaker than outside if you install the antenna on the attic.