The writings of the Apostle Paul exists as a collection known as the Pauline corpus, making up a significant portion of the New Testament writings. Regarding this collection of Paul’s writings, F.F. Bruce writes in his book The Canon of Scripture:
We do not know by whom or in what place the first edition of Paul’s collected letters was produced. C. F. D. Moule has suggested that it was Luke’s doing: “it is entirely in keeping with his historian’s temperament to collect them.” As for the place, Ephesus, Corinth and Alexandria have been suggested. The suggestion of Alexandria has been supported by the consideration that the editorial care devoted to the forming and publishing of the collection is entirely in line with the traditions of Alexandrian scholarship; on the other hand, the traditions of Alexandria lay right outside the sphere of Pauline Christianity.
What is important is this: from the early second century onward Paul’s letters circulated not singly, but as a collection. It was as a collection that Christians of the second century and later knew them, both orthodox and heterodox. The codex into which the letters were copied by their first editor constituted a master-copy on which all subsequent copies of the letters were based. There are relatively few variant readings in the textual tradition of the Pauline corpus – the time when the letters still circulated singly.
The oldest surviving copy of the Pauline corpus is the Chester Beatty manuscript P46, written about AD 200. Of this codex 86 folios are extant out of an original 104. It evidently did not include the three Pastoral Epistles (1 and 2 Timothy and Titus); on the other hand, it did include Hebrews, which comes second in its sequence of letters, between Romans and 1 Corinthians.
F.F. Bruce. The Canon of Scripture. (Downers Grove: IVP, 1988), 130.