Schematic, per the sch suffix, for the Audio Pro Ace Bass B2-70 subwoofer. The 546 KB PDF covers the built-in amplifier, likely including the ACE-Bass circuitry, Audio Pro's method of using the amplifier to extend bass from a small enclosure. It is useful for tracing the input, crossover and power amplifier stages, checking supply voltages and locating faulty output devices or protection parts. Confirm that the board markings in your unit match the drawing, as Audio Pro revised these modules over the years.
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.