Traditionally most Christians hold that the Biblical Canon has been closed because of the end of the Apostolic era. But if the Pope speaks from "Ex Cathedra", isn't that stating his teaching is similar/equal to the Apostolic authority keeping the Canon open?
The idea of a complete and clear-cut canon of the New Testament existing from the beginning, that is from Apostolic times, has no foundation in history. - Cf. Canon of the New Testament | New Advent.
A Primer on the Church's Infallibility
1) The Church's competency is faith [what we believe] and morals [how we are to act/behave in accordance with what we believe].
2) [Cf. Mt 28:20] All the Church was commanded to teach as to faith and morals by Jesus Christ is what is termed as the divinely revealed Sacred 'deposit of faith' (the depositum fidei) [= Holy Tradition + Sacred Scripture].
[This] Divine Revelation was given in its entirety to Our Lord and His Apostles. After the death of the last of the twelve it could receive no increment. - Cf. Revelation | New Advent.
3) [Cf. CCC 2035] Christ's charism of infallibility to his Church extends as far as does the deposit of divine Revelation; it also extends to all those elements of doctrine, including morals, without which the saving truths of the faith cannot be preserved, explained, or observed. What this simply means is that the Church cannot err in what she teaches as to faith or morals, for she is our infallible guide in both. [Cf. Penny Catechism 100].
4) [Cf. Infallibility | New Advent] The organs of infallibility in the Church are:
a) the bishops dispersed throughout the world in union with the Holy See;
b) ecumenical councils under the headship of the pope; and
c) the pope himself separately.
1) In the Constitution Ineffabilis Deus of 8 December, 1854, Pius IX pronounced and defined that the Blessed Virgin Mary "in the first instance of her conception, by a singular privilege and grace granted by God, in view of the merits of Jesus Christ, the Saviour of the human race, was preserved exempt from all stain of original sin." This was before:
2) The Vatican Council [1869-1870] which defined as "a divinely revealed dogma" that "the Roman Pontiff, when he speaks ex cathedra — that is, when in the exercise of his office as pastor and teacher of all Christians he defines, by virtue of his supreme Apostolic authority, a doctrine of faith or morals to be held by the whole Church.
3) By promulgating the Apostolic Constitution Munificentissimus Deus, 1 November, 1950, Pope Pius XII declared infallibly that the Assumption of the Blessed Virgin Mary was a dogma of the Catholic Faith.
Plain conclusions readily follow from above.
Clearing confusion in the question and answering it
When the Pope speaks ex cathedra according to the dogma of Vatican Council I, he can only do so only on those things that are already in the handed down deposit of divine Revelation which the Church in his generation has received through the ages from the Apostles or on all those elements of doctrine, including morals, without which the saving truths of the faith cannot be preserved, explained, or observed.
Councils have already determined the Sacred Scripture part of the 'deposit of faith' (the depositum fidei), no future organs of infallibility in the Church, pope included, can undo this.
Holy Scripture as the Church has held it and how the Church inteprets it is a matter of faith:
I also accept the Holy Scripture according to that sense which holy mother the Church hath held, and doth hold, and to whom it belongeth to judge the true sense and interpretations of the Scriptures. Neither will I ever take and interpret them otherwise than according to the unanimous consent of the Fathers. - Professio fidei Tridentina, Creed of Pope Pius IV [from the link, one of the four authoritative Creeds of the Catholic Church].
First of all, and purely as an incidental note, most Christians are Catholic (though that's just barely true).
To allow that the Pope can speak infallibly is different from "keeping the canon open", that is, allowing the possibility of additional books of Sacred Scripture. I cannot find anywhere in Church documents an official statement to the precise effect that "the canon of Scripture is closed". However, the fourth session of the Council of Trent (1546) did declare a list of the recognized books of Scripture:
The books that are received by this Synod. ... are as set down here below: of the Old Testament: the five books of Moses, to wit, Genesis, Exodus, Leviticus, Numbers, Deuteronomy; Josue [Joshua], Judges, Ruth, four books of Kings [in modern times I and II Kings are combined, and III and IV Kings are combined], two of Paralipomenon [Chronicles], the first book of Esdras [Ezra], and the second which is entitled Nehemias [Nehemiah]; Tobias [Tobit], Judith, Esther, Job, the Davidical Psalter, consisting of a hundred and fifty psalms; the Proverbs, Ecclesiastes, the Canticle of Canticles, Wisdom, Ecclesiasticus, Isaias, Jeremias, with Baruch [these two are separate books in the current Old Testament canon]; Ezechiel, Daniel; the twelve minor prophets, to wit, Osee [Hosea], Joel, Amos, Abdias [Obadiah], Jonas, Micheas [Micah], Nahum, Habacuc, Sophonias [Zephaniah], Aggaeus [Haggai], Zacharias, Malachias; two books of the Machabees, the first and the second. Of the New Testament: the four Gospels, according to Matthew, Mark, Luke, and John; the Acts of the Apostles written by Luke the Evangelist; fourteen epistles of Paul the apostle, (one) to the Romans, two to the Corinthians, (one) to the Galatians, to the Ephesians, to the Philippians, to the Colossians, two to the Thessalonians, two to Timothy, (one) to Titus, to Philemon, to the Hebrews; two of Peter the apostle, three of John the apostle, one of the apostle James, one of Jude the apostle, and the Apocalypse of John the apostle.
The Council, as I said, did not specifically state that these were all the books of Scripture there ever could be; however, neither did it state that there could be any more. Further, the Church does distinctly state that there is to be no further "public revelation" (that is, no new information on the life and teaching of Christ, or the nature of God:
Jesus perfected revelation by fulfilling it through his whole work of making Himself present and manifesting Himself: through His words and deeds, His signs and wonders, but especially through His death and glorious resurrection from the dead and final sending of the Spirit of truth. ... The Christian dispensation, therefore, as the new and definitive covenant, will never pass away and we now await no further new public revelation before the glorious manifestation of our Lord Jesus Christ.
(Dei Verbum section 4)
It would seem, therefore, that there can be no further "new books" of Scripture, that is, no inspired work to be included in the Old or New Testament.
What then does the infallibility of the Pope imply? Quite straightforwardly, the dogmatic constitution Pastor Aeternus fixing the infallibility (under certain circumstances) of the pope states:
The Holy Spirit was not promised to the successors of Peter that by His revelation they might make known new doctrine, but that by His assistance they might inviolably keep and faithfully expound the Revelation, the Deposit of Faith, delivered through the Apostles.
What the pope does, therefore, is not add anything new and different, but simply apply and restate definitively things that have always been part of the Revelation (most often the Sacred Tradition) of the Church.
Though the Pope, as the Successor of St Peter, is the spiritual descendant of the Apostles, and can speak with infallible authority, this authority does not imply the ability to add new books to the canon of Scripture.