Documentation for Apple logic board 820-3298, revision A, packed in a 2.44 MB RAR archive. The name does not give a model or list of contents, so open it to see whether a schematic, a boardview or both are included. Apple's 820-xxxx numbers and revision letters are printed on the PCB and both should match. Use the files to follow the power path, check enable signals, find components by designator and measure expected voltages on rails while chasing a fault.
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.
7-ZipWinRARThe Unarchiver (macOS)