Error in converting color pdf into grayscale pdf in Linux - 'File has errors...'

I have been trying to convert a color pdf to a grayscale pdf prior to printing to avoid the color printing costs. I am using Debian and ImageMagick commands:

convert -monochrome swp574.pdf output2.pdf

convert -colorspace GRAY swp574.pdf swp574GRAY.pdf

pdf2ps swp574.pdf output2.pdf

and I get this response on the terminal each time.

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.

How can I get over this problem?

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