The Act of Union of the Kingdom of Romania with Bessarabia, Bucovina, Transylvania, Banat, and the Romanian Regions of Hungary was ratified during the Paris Peace Conference. All documents from the National Archives of Romania.
Material of the Romanian delegation propaganda in Paris:

Voevod Alexandru (personal fond), ds. 115.
This booklet consists of the Resolution of Sfatul Țării, the regional parliament of Bessarabia (conditional and later unconditional union of Bessarabia), the Resolution of the General Congres of Bucovina in Cernowitz (in addition, the consent of polish and german nations in Bucovina), the Resolution of the Great National Assembly in Alba Iulia and the Royal Decrees for both latter unifications.
This is Mary, Queen of Romania.

Regală, Regina Maria. Personale. Memorii, ds. III 136_1922, f. 27v
In March 2019 she paid a visit to Georges Clemenceau, in Paris, to (successfully) support the Romanian interests at the Peace Conference.
Decree by which Ferdinand, King of Romania, empowers Alexandru Vaida Voevod,
president of the Council of Ministers, with the full authority to represent the country at
the Peace Conference in Paris (January 10th, 1920)

Voevod Alexandru (personal fond), ds. 118
Alexandru Vaida Voevod (1872-1950) was a supporter and promoter of the union of
Transylvania with the Romanian Kingdom.
The National Archives of Romania, The National Historical Central Archives Service, Documente
fotografice, I 6664.

fotografice, I 6664.