Schematic PDF for the Apple logic board numbered 820-2533. The file name gives only the board number, with no model name or revision letter. Apple uses the 820-xxxx pattern for its logic boards, so matching that number to the one silkscreened on the PCB is the key check. The diagrams help trace power rails, identify the regulators and switches in the power path, and follow signals during fault finding. It opens in a normal PDF reader. Revisions may differ, so compare part designators with the physical board.
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 @hrtechno. Enginerys does not host this file.
How to open a .pdf file
The standard format for schematics, block diagrams, power-sequence charts and service manuals.