我一直在尝试在打印之前将彩色 PDF 转换为灰度 PDF，以避免彩色打印成本。我使用的是 Debian 和 ImageMagick 命令：
convert -monochrome swp574.pdf output2.pdf
convert -colorspace GRAY swp574.pdf swp574GRAY.pdf
pdf2ps swp574.pdf output2.pdf
我每次都会在终端上得到这个响应。
File has circular references in resource dictionaries.
**** File has circular references in resource dictionaries.
**** This file had errors that were repaired or ignored.
**** Please notify the author of the software that produced this
**** file that it does not conform to Adobe's published PDF
**** specification.
我怎样才能解决这个问题？