Reference document of 1.20 MB in PDF form whose name, SDC, most likely refers to the SD memory card, although the file name gives no maker or version. Documents of this kind describe the card pinout, SD and SPI bus modes, commands, registers and timing. They help when diagnosing card readers, checking card-detect and write-protect lines or implementing a driver on a microcontroller. Open it to verify exactly which specification and edition it covers before relying on command or timing details.
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 @datasheet. 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.