Schematic PDF for Apple logic board 820-00840, revision A, 6.68 MB in size. It documents power rails, the power-on sequence, SMC-related signals and component values, making it the reference for working out why a board will not start or which rail is missing. Apple's 820-xxxxx numbers identify the logic board rather than the Mac model, so read the number off the PCB to confirm compatibility. It works best alongside the matching boardview to physically locate the parts named in the drawing.
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 .pdf file
The standard format for schematics, block diagrams, power-sequence charts and service manuals.