Zip archive filed under Apple logic board 820-2796, a 1.27 MB package whose name gives only the board number, with no model, revision or contents listed. Archives named this way usually hold a schematic PDF, a boardview, or both, for one MacBook-family logic board. Open it and check what is inside before relying on it. Apple 820-series numbers identify the main logic board, and the number silkscreened on the PCB must match exactly. Revision differences can move components and change rail names.
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.