The Lord’s Supper is a sacrament in the Presbyterian Church. We believe that the Holy Spirit is present in the elements but do not believe in transubstantiation. Communion is open.
You must be speaking of the Presbyterian USA church. This is from the PCA Book of Church Order:
Since, by our Lord's appointment, this Sacrament sets forth the Communion of Saints, the minister, at the discretion of the Session, before the observance begins, may either invite all those who profess the true religion, and are communicants in good standing in any evangelical church, to participate in the ordinance; or may invite those who have been approved by the Session, after having given indication of their desire to participate. It is proper also to give a special invitation to non-communicants to remain during the service.
"Evangelical Church" does not include the Roman Catholic Church. From a definition I found: During the Reformation, Martin Luther adapted the Greek term, dubbing his breakaway movement the evangelische kirche, or “evangelical church”