The Book of Hebrews fall under general epistles. Other General epistles include (James, 1Peter, 2Peter, 1John, 2John, 3John and Jude).
The book of Hebrews circulated independently for a long period of time before been accepted into the New Testament.
I choose the stance that Hebrews fall under the General epistles because of the following reasons.
1) NO CLAIM OF AUTHORSHIP
The anonymity of the book of Hebrews does not seem to be an accident. The first sentence of Hebrews (Hebrews 1:1-4), which is where Apostle Paul would normally identity himself as the author, if Apostle Paul wrote the book of Hebrews why did he not claim authorship of the book as he does in every letter he wrote? Usually in most of the Pauline epistles, Paul claims authorship by introducing himself as a “Bondservant of Jesus Christ” or “An Apostle of Jesus Christ”.
2) REFERRING TO JESUS AS HIGH PRIEST
Apostle Paul could not have been the author of the book of Hebrews because no letter of Paul refers to Jesus as High Priest. This is an important theological difference between the letters of Paul and Hebrews.
The style and language of the book of Hebrews are different from Apostle Paul’s other letters.
3)THE AUTHOR WAS SOMEONE WHO REMEMBERED THE APOSTOLIC TEACHINGS
It is suggested that Apostle Paul could not have been the author because the author of Hebrews includes himself among those to whom the gospel was confirmed by those who heard Jesus Christ(Hebrews 2:3) . Apostle Paul however argued strongly in Galatians that he had not recieved the Gospel from anybody but directly from God (Galatians 1:11-12).
No other book in the New testament ties together old testament history and practices with the life of Jesus Christ as thoroughly as the book of Hebrews. Just as Jesus taught that the old testament was fulfilled in Himself. So the author of Hebrews taught that the old covenant was brought to completion in the new covenant . The book of Hebrews also shows that because the old covenant has been fulfilled in the new covenant , the new covenant is actually better. The new covenant was made superior by the ministry of Jesus Christ.