Depends where it is. I wouldn't buy it online. If, for example, I somehow chanced upon a copy of a Challenger 2 manual at a militaria fair, would I buy it? Yes. Partly to take it out of circulation, partly because Im interested in the vehicle. I would display common sense where I displayed it though, which would be to nobody else. The only manuals ive ever copied have been of vehicles out of circulation in the British Army.
I once bought a manual that showed the TOE of 22 SAS Regiment at a shop in Evesham. That was only 10 years old at the time I bought it.. Ive never shared it anywhere, just on the off chance it wasnt off the restricted list. Personally, I think I did the MOD a favour. Others perhaps wouldnt.