PDF document for the 2N3791 part, filed under schematics but by its name a component datasheet for a JEDEC 2N-series transistor; the RE suffix in the name is not explained and may mark a reissued or revised copy. At 224 KB it is likely a scanned or multi-page sheet. Technicians use such a datasheet to confirm pinout, package outline and maximum ratings before fitting a replacement. Any PDF reader opens it, and markings on the actual part should be compared with the sheet before choosing a substitute.
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.