Answered by Shaykh Amjad Rasheed
Is it permissible for one praying behind an imam to look in the mushaf (copy of the Koran) while his imam is reciting?

Yes, in our school, it is permissible for one praying, whether a follower or otherwise, to look in a mushaf and carry it during an obligatory or supererogatory prayer.
However, if he moves his hand and the like, in too many consecutive movements, to carry it and open it, then his prayer is invalidated. It is best for him not to do it in the first place, so that he can leave the differences of opinion of those who say that it invalidates the prayer by carrying the mushaf.
Amjad Rasheed
[Translated by Sr. Shazia Ahmad ]