ssh -i path-to-pem-file ubuntu@ip-address

Selects a file from which the identity (private key) for public
   key authentication is read.  The default is ~/.ssh/identity for
   protocol version 1, and ~/.ssh/id_dsa, ~/.ssh/id_ecdsa,
   ~/.ssh/id_ed25519 and ~/.ssh/id_rsa for protocol version 2.
   Identity files may also be specified on a per-host basis in the
   configuration file.  It is possible to have multiple -i options
   (and multiple identities specified in configuration files).  If
   no certificates have been explicitly specified by the
   CertificateFile directive, ssh will also try to load certificate
   information from the filename obtained by appending -cert.pub to
   identity filenames.