PDF circuit diagram of a mono tone control stage built around two transistors, as the name indicates. Files like this suit hobbyists and audio repairers who are building or servicing a simple bass and treble preamp ahead of a power amplifier. Any PDF viewer will open it. The name gives no brand or component values, so check part values, supply voltage and pinouts against the drawing itself. The (2) suffix suggests a duplicate copy, so compare it with other versions before you build.
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 @pdf_servicemanual. 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.