To those who believe Paul was the author of Hebrews, consider this.
2 Thessalonians 3:17
The
salutation of Paul with mine own hand,
which is the token in every epistle: so I write.
Paul said his salutation validates the epistle he writes. The book of Hebrews doesn't contain Paul's salutation.
Let me give you examples:
Galatians 1:3
Grace be to you and peace from God the Father, and from our Lord Jesus Christ,
Ephesians 1:2
Grace be to you, and peace, from God our Father, and from the Lord Jesus Christ
Colossians 1:2
To the saints and faithful brethren in Christ which are at Colosse: Grace be unto you, and peace, from God our Father and the Lord Jesus Christ.
Philemon 1:3
Grace to you, and peace, from God our Father and the Lord Jesus Christ.
Paul validates all his written epistles with his salutation. According to Paul's statement, he didn't write the book of Hebrews. Why, because it doesn't contain Paul's salutation.