Is it legal to drop off a subpeona without the persons signature?

Yes and most frequently it is, because the person receiving it will not be happy to get it and will not co-operate by signing for it. If the people knew that service would be ineffective if not signed for, no one would sign. The proof that the service was made is in the form of an affidavit by the person serving the subpoena. I served enough of them myself to see what happens.