Small PDF listed as a schematic, though the name 2SA777 with suffix E reads like a discrete semiconductor part number in the Japanese 2SA style rather than a board designation. At 50.5 KB it is most likely a short datasheet with pinout, ratings and characteristic curves. A technician uses such a sheet to confirm the pin order of a transistor before replacing it, compare voltage and current limits against a substitute part, and check marking codes found on the board. Any PDF reader opens it.
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.