For the 2SK664 transistor, this 34.9 KB PDF is indexed as a schematic but, judging by the JIS 2SK naming used for N-channel field-effect transistors, it is a datasheet for a single component. Typical content includes package drawing, pinout and an absolute maximum ratings table. Technicians rely on such documents to verify pin order before fitting a part, check whether a shorted device matches its rated limits, and select an alternative with compatible voltage, current and package. Open it with any PDF reader.
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.