remoak talks sense.
A precautionary shutdown, unless accompanied with further nasty problems, does not place the aircraft in a "condition of being threatened by serious and/or imminent danger." Perhaps in this case the commander was aware of some catastrophic hydraulic / fuel leak or fire etc of which we are unaware?
In the three in-flight precautionary shutdowns in two engined aeroplanes I have made, I have declared a PAN because the situation warranted what was effectively the definition of PAN. MATS states ATC should consider 50% of remaining power units to be a full emergency. This definition does not exist in any pilot publication I have ever read so the perspective is not uniformly shared.
In the event of an unexpected failure, I would almost certainly use Mayday since there may be underlying, further unknown damage that puts me in a "condition of being threatened by serious and/or imminent danger."
The definition of both terms are published widely as remoak describes. I've never understood why some try to create their own definitions or assumptions of their meaning.
As an aside, I would always use Mayday when on one remaining power unit outwith the UK, simply because PAN is not often recognised there and there is no definitive alternative I have ever found.
As another poster mentioned, the Kestrel pilots have it easy in this regard. They have a printed SOP that demands they use Mayday. Here lies the Captain's privilege.
Silent Jumpseater -
I have not seen that written in any Boeing manual. Can you give me a reference? Are you perhaps confusing it with the requirement to "land immediately." That is not the same as putting out a Mayday.........