RAR package of 1.55 MB for Apple board 820-2301. Only the board number is in the name, so revision and contents remain unknown until extraction; a size like this usually means a schematic set. Apple prints 820-xxxx part numbers on its boards, so check that yours matches and note the revision letter. The drawings help trace power rails, read the startup sequence, find test points and isolate the component that shorts a rail or stops the machine from powering on.
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.