Large 12.0 MB RAR archive for Apple logic board 820-2838, much bigger than a typical single schematic, which hints at multiple files, images or a boardview, though the name lists nothing beyond the board number. Revision and model are not stated. Verify the 820 number and revision on the PCB before relying on it. Extract with WinRAR or 7-Zip, then use the contents to trace power rails, locate test points and components, and compare measured values against the reference during diagnosis.
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.