Apple iPhone 4S documentation packed in RAR format, roughly 3.8 MB. The name states only the model, so the actual contents, likely a logic board schematic or boardview, need to be confirmed after unpacking. Technicians rely on such files to trace signals between the baseband and application processor, check power supply lines, locate small components and find shorted capacitors. The 4S board differs from the iPhone 4, so verify the documents apply to your phone before trusting part references.
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 @cyberlanelectonik. Enginerys does not host this file.
How to open a .rar file
Compressed archive — the usual wrapper for a schematic + boardview pair or a BIOS + EC set.