This is to convert a pdf document (not pdf/a) into pdf/a:
gs -dPDFA -dBATCH -dNOPAUSE -dUseCIEColor -sProcessColorModel=DeviceCMYK -sDEVICE=pdfwrite -sPDFACompatibilityPolicy=1 -sOutputFile=output_filename.pdf input_filename.pdf
Hope this will help some one!