Small 41.6 KB PDF for the 2SC5388, a JIS-numbered NPN bipolar transistor; the file name states only the part number, so package, maker and application are not given. Most likely it is the datasheet, holding pin assignments, voltage and current maximums, gain ranges and outline drawings. These details help when testing junctions with a multimeter, tracing why a stage has no output, or choosing a cross-reference replacement. Verify the device marking on the board corresponds to this number before relying on the data.
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.