ZIP archive of about 1.33 MB containing the public design release for the Raspberry Pi Compute Module IO board, revision 1.2. Such packages generally provide schematics together with layout or manufacturing data intended to help developers build their own carrier boards; extract it to see the exact file set. Layout data opens in a PCB tool or Gerber viewer, schematics in a PDF reader. Use it for footprint references, connector placement and dimensions, and confirm your physical board is the V1.2 version.
The button opens the matching file on RM Schematics, or a source page if it is not catalogued there yet. The original attachment is from @boardview. Enginerys does not host this file.
How to open a .zip file
Compressed archive that every operating system opens without extra software.