At 214 KB, the 2SC3964 PDF is one of the larger files in this group, likely a full datasheet with characteristic curves for this JIS-numbered NPN bipolar transistor, though catalogued as a schematic. Only the part number is given in the name. Curves for gain, saturation voltage and safe operating area help judge whether a substitute will survive in the same circuit. Confirm the package and the abbreviated C3964 marking on the PCB before ordering or fitting a replacement part.
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.