A 39.3 KB PDF for the 2SC5490 component, which belongs to the 2SC range of NPN bipolar transistors under Japanese JIS numbering. The file is listed as a schematic, but its contents are probably the device datasheet. Typical sections are the package outline, pin identification, absolute maximum ratings and characteristic curves. It is useful for cross-referencing a burnt or shorted transistor, checking that pins are oriented correctly on the board, and judging whether a proposed equivalent can handle the same voltage and current.
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.