An archive of repair files for Apple hardware (board 820-2850). Shared as “820-2850.zip” — a .zip file, 2.13 MB.
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 @schematics_bios. Enginerys does not host this file.
How to open a .zip file
Compressed archive that every operating system opens without extra software.
Built-in OS tools7-Zip