Apple board 820-3024 repair documentation in a compact 1.25 MB zip, with no model or revision stated in the file name. The 820 prefix is Apple's logic board numbering, so the archive probably includes a schematic and perhaps a boardview. Technicians rely on these to follow the power path, confirm expected voltages, check components and locate a short on a given rail. Open PDFs with any reader and layout files with OpenBoardView. Confirm the board number printed on the PCB matches before diagnosing.
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.