The 2SD1724 part number is all the name gives, and the 145 KB PDF is tagged as a schematic, though a file named this way is normally a transistor datasheet. JIS-registered 2SD devices are NPN bipolar transistors for low-frequency service, common in power supplies, drivers and audio stages. Open it in any PDF reader to check the pin order, case style and absolute maximum ratings. Compare collector voltage, current and gain against the original part before choosing a substitute.
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.