Cardinal Jose Tolentino Calaca de Mendonca, the Vatican's chief librarian, told reporters that all researchers _ regardless of nationality, faith and ideology _ were welcome to request permission to use the Vatican's Apostolic Library, which will open the archive on March 2.
``The church has no reason to fear history,'' he told reporters.
Some Jewish groups and historians have said Pius, who was pope from 1939-1958, stayed silent during the Holocaust and didn't do enough to save lives. His defenders at the Vatican and beyond say he used quiet diplomacy and encouraged convents and other religious institutes to hide Jews.
Emeritus Pope Benedict XVI, a great defender of Pius, accelerated the process to open the archives ahead of schedule so that researchers could have their say. Pope Francis announced the archive would be ready March 2.
One of the historians who plans to be here for opening day is David Kertzer of Brown University, author of several books about Pius' predecessors and their relations with Jews. One about Pius XI, ``The Pope and Mussolini,'' won the Pulitzer Prize for biography in 2015.
In an email, Kertzer said the imminent opening of the Pius XII archives, and the light it will shed on the role played by the pope during the war, had ``generated tremendous excitement in the scholarly world, and beyond.''
``Much of historical importance will also become clearer for the postwar years, when the pope, among other challenges, worried that the Communist Party would come to power in Italy and played a crucial behind-the-scenes role in blocking it,'' he said.
Officials warned that the process of studying the millions of pages of documents from six different archives will be measured in years, not days, weeks or months, and will require patience.
``It is unthinkable for a researcher to come to the archives and either look for an easy scoop in a short time or write a book in just one year,`` stressed Monsignor Sergio Pagano, the prefect of the archive. ``We will need several years.``
The documentation includes the archives from the Pius secretariat of state _ the main organ of church governance, which includes the Vatican's foreign relations with other countries _ as well as those of the Vatican Congregation for the Doctrine of the Faith, or CDF, and the Vatican office responsible for mission territories.
The CDF documents, for example, include case files of priests disciplined for pro-Nazi political activity, said Monsignor Alejandro Cifres Gimenez, archivist at the doctrine office.
The mission archives, meanwhile, shed light on the ordinary life of the church outside the main centers of Catholicism in Europe and North America, such as the effects of the war in Japan, said the archivist for the mission office, Monsignor Luis Manuel Cuna Ramos.
In addition, one document he recalled was the first letter Mother Teresa wrote to Pius on March 1, 1950, asking for his approval of her new religious order in India, he said.
Jewish groups and historians have argued for years that the Vatican had no business moving forward with Pius' beatification cause until the Vatican's full wartime archives were opened. They have also asked that any beatification be put off until the generation of Holocaust survivors have died.
The American Jewish Committee, which voiced such appeals, welcomed the opening.
``We trust that the independent scholarly review of these archival materials will provide greater clarity as to what positions and steps were taken during this period by the Holy See, and help resolve the persistent debates and controversy in this regard'', said Rabbi David Rosen, in charge of the group's interreligious affairs.
He said the ``necessary transparency'' would also enhance already strong Catholic-Jewish relations.
Benedict moved Pius one step closer to possible sainthood in December 2009, when he confirmed that Pius lived a life of ``heroic`` Christian virtue. All that is needed now is for the Vatican to determine a ``miracle`` occurred.
Pope Francis said in 2014 that the miracle hadn't been identified, suggesting that the process would remain on hold, at least for now.