Stored under schematics, this 171 KB PDF is named 2SD1407A and is most probably the datasheet for that transistor variant, the A suffix distinguishing it from the base 2SD1407. The Japanese JIS 2SD prefix identifies NPN bipolar transistors for low-frequency circuits. A technician checks it for pinout, package outline and absolute maximum ratings when testing junctions or choosing a replacement. Confirm the package matches the original, and search the board for the shortened D1407A marking.
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 .pdf file
The standard format for schematics, block diagrams, power-sequence charts and service manuals.