RAR archive of 1.74 MB for Apple logic board 820-2936, revision A according to the name; the lb prefix likely refers to logic board. The exact contents are not listed, but files of this kind usually hold a boardview, schematic or both. 820- numbers are Apple's logic board part numbers, printed on the PCB near the edge. Extract the files and check the type before use, then rely on them to trace power rails, locate components and probe test points.
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 @cidschematicandboardviewofficial. 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)