Paul, for 93% of his books, had someone write down his words for him. He would then include a handwritten salutation to close each epistle (see 2Thessalonians 3:17, Galatians 6:11, 1Corinthians 16:20 - 21, etc.). The below list, in chronological order of his books, reveals (where possible) the Christians who aided Apostle Paul's ministry by being his secretary.
Thessalonians
The books of 1 and 2 Thessalonians, though not recorded in either writing, were likely dictated by Paul to Timothy or possibly Silas (The Bible Illustrator, Jamieson-Fausset-Brown Commentary).
Both Silas and Timothy, when these two books were written, had recent experience serving those in Thessalonica (Acts 17:10, 1Thessalonians 3:1 - 5). They were also with Paul in Corinth when he wrote his letters to the church in 50 and 51 A.D. (1Thessalonians 1:1, 2Thessalonians 1:1).
Galatians
Galatians is the only one of Paul's books to have been written entirely by him (Matthew Henry's Commentary). It is also unique in that it is void of his usual praise and thanksgiving for believers (see 1Corinthians 1:3 - 9).
"Apparently the only Epistle that Paul wrote with his own hand was that addressed to the Galatians (Galatians 6:11). He usually dictated his Epistles to a scribe." (Holy Bible, a Faithful Version, page 46).
Corinthians
The Bible reveals that Paul's book of 1Corinthians was dictated to and written by four individuals!
My love be with you all in Christ Jesus. Amen. The first epistle to the Corinthians, written . . . by Stephanas and Fortunatus and Achaicus and Timothy (1Corinthians 16:24, HBFV throughout).
Paul dictated 2Corinthians to his fellow co-workers Titus and Luke.
The grace of the Lord Jesus Christ, and the love of God, and the fellowship of the Holy Spirit be with you all. Amen. The second epistle to the Corinthians, written from Philippi of Macedonia by Titus and Luke (2Corinthians 13:13).
Romans
The book of Romans tells us who Apostle Paul used to record his words to the church in Rome.
I, Tertius, who wrote this epistle, salute you in the Lord (Romans 16:22).
Hebrews
Paul wrote Hebrews in his first year as a prisoner in Rome (Acts 28) in 61 A.D. The book, unfortunately, does not say who recorded the apostle's words. That said, it was highly likely his friend Luke who acted as his copyist as he had traveled with the apostle to the empire's capital.
"There is no question that Paul wrote all the Epistles that bear his name. On the other hand, there has been a great deal of debate about the authorship of Hebrews.
"The style of writing in Hebrews is very close to Luke’s style, thus some have concluded that Luke is the author. The evidence, however, reveals that the book of Hebrews originated with the apostle Paul - who undoubtedly used Luke as his scribe." (Holy Bible, a Faithful Version, 2nd Ed., page 53).
Ephesians
Paul's letter to the church in Ephesus is considered one of his "prison epistles" as it was written during his first incarceration in Rome. Other books in this set, penned from 61 to 63 A.D., include Philippians, Colossians and Philemon.
Ephesians does not reveal who wrote this book. A man named Tychicus (Ephesians 6:21), who highly likely delivered the letter, may have also been the one to record it.
As with all the prison epistles, there were several people available at various times who could have copied down Apostle Paul's words. Other than Tychicus, these included Timothy (Philippians 1:1), Aristarchus, Luke, Demas, Onesimus, Mark and Justus (Colossians 4:9 - 14), Epaphras (Colossians 1:7), Epaphroditus (Philippians 4:18) and others.
Philippians
Philippians does not record who wrote down Paul's words. That said, Timothy was with the apostle at the time (Philippians 1:1) as well as a man named Epaphroditus.
The church at Philippi, in an act of amazing generosity, sent Epaphroditus to Rome with financial help for the apostle. Paul was incredibly grateful for the gift.
But I have all things and abound. I am full after receiving from Epaphroditus the things you sent, a sweet-smelling savor, a sacrifice acceptable and well pleasing to God (Philippians 4:18, see also 2:25 - 30).
It is highly likely that either Timothy or Epaphroditus was the scribe Paul used for this book.
Colossians
Paul's letter to Colosse does not mention who wrote down its words. The number of people the book lists in Rome who could have done so, however, is the most of any of the prison epistles!
Timothy was with Paul in Rome when he wrote this book (Colossians 1:1). Those also in the empire's capital included Tychicus, Onesimus, Aristarchus, Mark, Justus, Epaphras, Luke and Demas (Colossians 4:7, 9 - 12, 14).
Philemon
Paul's letter to his friend Philemon does not mention who wrote down its words. This short epistle, however, lists the same men as Colossians (minus Tychicus and Justus) who could have provide their service as the apostle's secretary.
Paul, the prisoner of Christ Jesus, and Timothy, a brother, to Philemon . . . I beseech you for my son, Onesimus, whom I begot in my bonds . . .
Epaphras, my fellow prisoner in Christ Jesus, salutes you; As do also Mark, Aristarchus, Demas and Luke, my fellow workers (Philemon 1:1, 10, 23 - 24).
1Timothy
The Apostle Paul wrote the book of 1Timothy in 63 A.D. after he was acquitted and released by the Romans in Rome.
Paul does not mention anyone being with him in Nicopolis during the time he composed 1Timothy. Luke, however, who was with him during his Roman imprisonment, was possibly still with him in Nicopolis. If this was the case he likely was the person who copied down this book.
Titus
Paul, after he was released from Roman imprisonment, wrote to his friend Titus in 63 A.D. His letter was written from Nicopolis in Macedonia where he planned to spend the winter.
All of those with me salute you. Salute those who love us in the faith. Grace be with all of you. Amen. To Titus, chosen to be first overseer of the Cretan church, written from Nicopolis of Macedonia (Titus 3:15).
Two men, Artemas and Tychicus, were with Paul at Nicopolis. It is likely that one of them, or another of the unnamed individuals with the apostle (Titus 3:15), wrote down this letter to Titus.
2Timothy
In January or February of 68 A.D., just before his martyrdom, the Apostle Paul from Rome writes his last Biblical book. He does not, unfortunately, directly name who wrote down his words.
The Lord Jesus Christ be with your spirit. Grace be with you. Amen. The second epistle to Timothy, who was chosen first overseer of the church of the Ephesians, written from Rome when Paul was brought a second time before Caesar Nero (2Timothy 4:22).
2Timothy, however, does mention that Paul's friend Luke was with him. It also reveals that several brethren, likely from the local fellowship, also visited the apostle.
Only Luke is with me. Get Mark and bring him with you, because he is profitable to me for the ministry of the Word . . . Eubulus salutes you, and Pudens, and Linus, and Claudia; and all the brethren salute you (2Timothy 4:11, 21).
It is almost certain that one of the people mentioned above wrote down Paul's last book.
Conclusion
The Apostle Paul used a wide range of fellow laborers to write his books. They included two gospel writers (Mark and Luke) as well as Onesimus, Silas, Timothy, Titus and Tychicus. They also likely included at least nine other named individuals. Each one played a significant role in spreading the good news of God's kingdom to the world.