Listed under schematics, this 168 KB PDF is named after the 2SC5755 part number only. In JIS/JEITA terminology the 2SC prefix marks an NPN bipolar transistor for high-frequency applications, so the file is most probably its datasheet, likely with characteristic graphs given the size. Technicians consult it for maximum ratings, pin assignment and package dimensions when diagnosing a shorted transistor or selecting a substitute. Check that the fitted part has the same package and marking.
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.