Apple logic board 820-3435 repair files compressed into a 1.25 MB zip archive. Nothing beyond the board number is stated, so the model and revision need to be confirmed from the PCB marking. Archives of this kind normally hold the schematic and sometimes a boardview, which together let a technician trace power rails, verify component values and locate test points while hunting for faults. Use a PDF reader for the diagram and OpenBoardView or a similar program for layout files, and match the revision letter.
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.