Indexed as a schematic, this 91.5 KB PDF named 2SC4390 is most likely the datasheet of that NPN bipolar transistor. In the Japanese JIS scheme, 2SC denotes NPN types of the high-frequency class, though they appear in many general circuits. The name provides no other details. Use the document to confirm pins with a meter in diode mode, compare ratings and gain groups with possible equivalents and read the outline dimensions. Check the printed marking on the original part before relying on the sheet.
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.