Compressed file for Apple board 820-3536, 1.35 MB, with no revision letter or model stated in the name. Its contents are likely a schematic, though this should be verified after unpacking. Apple 820- numbers designate the printed circuit board, silkscreened on the logic board, so compare it before use. A schematic like this helps a technician follow power sequencing, check rails during no-power diagnosis and identify components by reference designator when replacing parts.
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 @Mac_Shematic_Santale. 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.