The 2SC4121 part number is the whole name of this 89.5 KB PDF; although typed as a schematic, it should be the datasheet for that NPN bipolar transistor, as the Japanese JIS 2SC prefix indicates. Expect C4121 as the marking on the component. The document supports pinout identification for diode-mode testing, review of maximum ratings and selection of a suitable equivalent. No manufacturer, gain class or edition is given, so check the package outline and any suffix letter on the board part before ordering or fitting a replacement.
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.