License. 7-Zip is free software with open source. The most of the code is under the GNU LGPL license.
The story typically follows a standard "lost media" or "dark web" trope:
: Most instances of petitmary.7z found in the wild were actually "zip bombs" (files that expand to an enormous size to crash a computer) or simple malware disguised as the legend to trick curious users.
: It is widely believed to be an imitation of the "Smile Dog" or "Mariana's Web" style of creepypasta, designed to create a sense of digital dread.
: Upon extraction, the archive is said to contain a series of highly distorted images of a young girl ("Mary") and a single, low-bitrate audio file.