PDF of 56.2 KB named 2SD1391, a JIS designation for an NPN low-frequency bipolar transistor, and presumably its datasheet although indexed under schematics. Expect a ratings table, hFE and saturation data and a package drawing. It helps when testing junctions with a meter, confirming lead order and choosing an equivalent with adequate collector voltage and power ratings. The file name carries only the part number, so verify the maker and case of the device on the board.
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.