Boardview file for the Apple logic board 820-02536, in the .brd layout format. It shows component placement on both sides of the PCB, with part designators, net names and pin connections. That makes it handy for finding test points, measuring to ground when hunting a short and locating a part named in the schematic. The 820-xxxxx pattern is Apple's own board numbering. Open it with an Apple-capable viewer such as OpenBoardView, FlexBV or a similar tool, and check that the number on the board matches.
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 .brd file
The most common boardview extension. Several unrelated formats share it, which is why one viewer opens a .brd and another refuses it.