RAR archive for the Apple logic board numbered 820-3115, about 2.21 MB in size. The name carries no revision, model or content list, so open it to see whether it holds a schematic PDF, a boardview file or both. Apple uses the 820-xxxx pattern for its own logic board part numbers, printed in the copper or silkscreen of the PCB. Schematics help trace power rails and enable signals, while a boardview lets you locate components and test points in a viewer such as OpenBoardView. Confirm the number on your board before relying on it.
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.