Question

Do offices issue certificates to the elected candidates? Who signs the certificates?

Answer 1

yes the office issue certificate to the candidate and it is signed by election commissioner

Answer 2

Answer:

After the election is over, the elected or the winning candidates are issued by the Office of the Election Commissioner or its representative, and it often turns out to be the returning officer.  

Election Commission gives printed certificates for the candidates elected for the Loksabha seat.

Again all details of the candidate are sent by the returning officer to the Office of the Joint Secretary.