Repair package for Apple logic board 820-3115 with revision letter b, stored as a 1.59 MB zip. The name does not mention a model, so confirm the machine from the PCB marking. Inside you would typically find a schematic and possibly a boardview, used for tracing power sequencing, measuring rails, locating test points and narrowing down shorts. Schematics open in PDF readers while boardview files need OpenBoardView, FlexBV or similar. Check the revision letter, as revision B may differ from earlier boards.
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 @notebookschematic. Enginerys does not host this file.
How to open a .zip file
Compressed archive that every operating system opens without extra software.