They provide good detail about the software they used.

"The major software tools used for the Offshore Project were NUIX of Sydney, Australia, and dtSearch of Bethesda, Md. NUIX Pty Ltd provided ICIJ with a limited number of licenses to use its fully featured high-end e-discovery software, free of charge. The listed cost for the NUIX software was higher than a non-profit organization like the ICIJ could afford, if the software had not been donated."

Securiing their communications proved more difficult than free-text search, however:

"The project team’s attempts to use encrypted e-mail systems such as PGP (“Pretty Good Privacy”) were abandoned because of complexity and unreliability that slowed down information sharing."