Hi, Tony,
I, the principal tenant of the above property confirm that I have received the following keys from the Landlord (Adam Malik).
Yes, the article is correct there.
However, it is usual for the parties to a contract to be preceded by the zero article once they have been defined at the beginning of the document. This has nothing to do with grammar but is only a typical practice in legalese:
This lease is entered into by Tony C (Tenant) and Adam Malik (Landlord).
[...] Tenant hereby promises to return the key to Landlord at the end of the lease.