Paul generally made a point of naming himself as the author of the books he wrote, apparently because he was concerned about forgeries in his name. The writer of Hebrews does not name himself.
Paul repeatedly states that he received the Gospel directly from Jesus himself, whereas the writer of Hebrews places himself one step lower in the chain, stating that he heard it from those who heard it from Jesus. (...ὑπὸ τῶν ἀκουσάντων)
The writers of the NT were a mixed bag, with the writer of Revelation being barely literate, (At least in Greek) clear up to the writer of Hebrews, which is arguably the most sophisticated Greek in the NT. The writings that are definitely authored by Paul really aren't at this level.