First of all Paul was very likely to have been a historical figure unlike most of the charactors in the new testiment. He is thought to have had a limited education judging from his primative writing style which is why we know that 6 of the 13 letters attributed to Paul were actually later forgeries.

He is thought to have had many of his letters writen for him. They would have been likley written on velum scrolls. Later, by many many years, copies of copies were then put into codices. While Christianity does like to take credit for innumerable things they did not do, the codices are actually attributed to the Christian movement. The reasons for this is unknown, but scholars believe that they simply found it easier to transport and edit one page at a time rather than a scroll that could be 8 ft. In length. So books as we know them today are actually of Christian orgin.

article talking about the addition of the chapter and verse might help

[new.wartburgproject.org]

might be a reference to get more information.

[en.wikipedia.org]

That was a time of transition, could have been either. But since they are letters and long ones at that, and if we take that literally, a big if, then perhaps not codices. The Romans used many writing media, including folded wooden cards, multiple ones of which could be used. Fail to see though why it is interesting.