دَخَلَ الْبَاحِثُ يَحْمِلُ الْوَثَائِقَ.
The researcher entered carrying the documents.
“Yaḥmilu al-wathāʾiqa” is an affirmative imperfect clause describing the researcher's state at the time of entering. The owner of the state is “al-bāḥithu,” referred to by the implicit subject of “yaḥmilu.” With this pronominal link, the usual verbal circumstantial pattern is expressed without a wāw of ḥāl.
In “dakhala al-bāḥithu wa-huwa yaḥmilu al-wathāʾiqa,” “huwa” can function as the subject of a nominal clause whose predicate is verbal. That is a different analysis from simply putting wāw before an affirmative verbal ḥāl clause, and the distinction explains why the two expressions should not be classified mechanically.