Games Workshop (and by extension Forge World) aggressively protects its IP. There is no official, legal PDF of this book available for purchase. Warhammer Digital has never released the back-catalogue of Imperial Armour as PDFs. You cannot buy it on DriveThruRPG or the GW site. Any free PDF you find is, by definition, a scanned, unlicensed copy.
The second edition of Imperial Armour Volume Three: The Taros Campaign PDF is expected to include: Games Workshop (and by extension Forge World) aggressively
Most circulating PDFs of the second edition are user-scanned. Quality varies wildly: Games Workshop (and by extension Forge World) aggressively