A 2.65 MB zip labelled with Apple logic board number 820-2494 and nothing else, so the model, revision and exact contents are not stated. Such archives typically bundle a schematic and possibly a boardview for that logic board. They are used for diagnosing power faults, locating parts and identifying test points for voltage or resistance checks. Apple uses the 820 prefix for main logic boards. Make sure the number on your PCB matches and that any revision noted inside agrees with the board before trusting values.
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 @notebookschematic. Enginerys does not host this file.
How to open a .zip file
Compressed archive that every operating system opens without extra software.