Catalogued as a schematic, this 156 KB PDF is named after the 2SD1906, a Japanese JIS part number, so it is most likely the component datasheet for that transistor rather than a board drawing. Under the JIS scheme the 2SD prefix marks an NPN bipolar transistor intended for low-frequency use. Technicians open such a sheet in any PDF reader to check pinout, package outline and maximum ratings before fitting a replacement or choosing a substitute, and should confirm the full marking on the original part, including suffix letters.
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.