Epistles to the Churches (Philippi, Colossian, Thessalonica) and Timothy, Titus and Philemon
Paul’s Epistle to the gentile believers at Philippians, Colossians,
Philemon and his instructions to Timothy and Titus do not contain any direct quotes from the scripture (old testament). The churches (Philippians, Colossians & Thessalonians) were made up of Gentile Believers most of whom would not know the scripture.
Philemon was a fellowlabor who had a church meeting in his house. The epistle was mainly to ask a favor of Philemon on the behalf of Onesimus.
The epistles to Timothy
and Titus were instructive on how to run a church, requirements for church leader and what was coming for the church among other things. Since the church (body of Christ – Eph 1:22-23) is not mentioned in the scripture he would not have anything to quote from them. There is no question that Paul authored each of these epistles, see verse 1:1 in each of them.