RAR archive of 4.85 MB for the Apple logic board 820-00840, revision A according to the name. The contents are not listed, but packages named after an Apple board number usually carry the schematic, a boardview file or both. Extract it with any RAR-capable tool and check what is inside. Before relying on the documents, compare the 820-00840 number and revision letter with the silkscreen on the PCB, because Apple reuses similar numbers across closely related boards.
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 @one5technology. Enginerys does not host this file.
How to open a .rar file
Compressed archive — the usual wrapper for a schematic + boardview pair or a BIOS + EC set.