Schematic in PDF form for a board identified only as T61 in the file name, which most likely refers to the ThinkPad T61 line, though the name gives no brand, board number or revision. At 2.09 MB it should contain the full circuit diagram set. Technicians use such drawings to trace power rails, follow signal paths between the chipset and peripherals, and identify components around a short or a dead rail. Any PDF reader opens it; compare the board number printed on the PCB with the title block before trusting part values.
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 @alischematics. 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.