Is it permissible for someone entrusted with distributing Zakat funds to open the sealed envelopes containing these funds and distribute them to a larger number of deserving poor people instead of distributing them as they are in the envelopes?

If the owner of the money grants the person full authorization to dispose of the money as he deems appropriate, then it is permissible for him to open the envelopes to distribute what is inside according to his discretion, unless he knows that the giver does not wish that or does not like anyone to be informed of the amount distributed, in which case he should distribute them without opening them.