RAR archive of 1.63 MB for Apple logic board 820-2936; the name gives no revision letter, model number or list of contents. The same board number is known from A1278 machines, but confirm against your own unit and the revision printed on the PCB. Expect a schematic and possibly boardview data inside; extract it with 7-Zip or WinRAR. Technicians use these files to trace the power-on sequence, read signal names between controllers and find where a shorted rail originates before lifting components.
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.