RAR archive for Apple board 820-2877, 1.58 MB in size, with only the board number in its name. No model, revision letter or contents are specified, so unpack it with 7-Zip or WinRAR to see what is inside. Apple uses the 820-xxxx pattern for its in-house logic boards, and the number is printed on the PCB. A schematic from such an archive helps measure expected voltages on each rail, read power-good and enable logic and pick replacement parts by value and designator.
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.