in this scenario I HIGHLY DOUBT it is illegal (but then again I'm not a lawyer)

if RAID1 is mirroring (two harddrive working in conjunction for redundancy)

Because the "logical storage drive" is seen as a single storage unit. when you install or download the PDF you are in principle downloading it to a single entity and using it from a single entity
It is how the underlying storage technology works.

it's not like the two harddrive is being used independently and another person have access to the PDF/data at the same time as you