At only 24.2 KB, the 2SC3989 PDF is a compact document, probably a short datasheet or summary page for this JIS-numbered NPN bipolar transistor despite being indexed as a schematic. The name states nothing beyond the part number. A small sheet like this usually still lists the key ratings, pin order and package, enough to identify base, collector and emitter during testing. Since it may be brief, cross-check critical limits against a fuller source and confirm the C3989 marking on the component itself.
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.