if one has a scanned copy watermarking could be done through software.... but I don't see any additional advantage over writing the purpose and signing and putting a date....so that it cannot be missued at a later date
whenever sharing copies ...write the purpose of it on the copy ...
for e.g. if for telephone connection ....write on it for Airtel Connection and sign there
if alternative there like for address passport copy also allowed then give that
rest is not in our hands