A 1.95 MB zip archive identified solely by Apple board number 820-3126. No model or revision letter is given, so verify the details against the board you have. Apple 820 part numbers refer to logic boards, and archives like this generally contain a schematic and sometimes a boardview. These help a technician trace rails, check the SMC and charging circuits, look for shorts and find components. Open the PDF in any reader and boardview data in OpenBoardView or FlexBV after extracting.
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.