RAR archive of 4.85 MB for Apple logic board 820-00840, revision A according to the suffix. The name does not say what is inside, but the size suggests a schematic, possibly with a boardview. Use it to trace power rails, find test points and diagnose faults on the main board. Check that both the board number and the revision letter on the silkscreen match, because revisions can change components or rail routing. Extract with any archiver that supports RAR.
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 @cyberlanelectonik. 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)