Diagram document for the Apple iPhone SE in PDF format, 2.10 MB in size; the name calls it a diagram and gives no board number or revision. Most likely it contains circuit schematics useful for following power rails, checking charging and backlight circuits and locating components connected to a suspect net. A regular PDF viewer opens it. Before measuring, verify which iPhone SE generation the device is, since the first and later SE models use completely different logic boards.
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 @schematic_mobiles. 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.