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I want to convert a PDF of text to black and white, and was wondering if there was any difference between

convert -monochrome in.pdf out.pdf

and

convert -colorspace GRAY in.pdf out.pdf
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The output with -monochrome is black and white without any colors inbetween while -colorspace converts the input into the given destination colorspace.

gm convert options:

-monochrome
transform the image to black and white

gm convert colorspace:

-colorspace <value>
the type of colorspace
...
Two gray colorspaces are supported. The Rec601Luma space is based on the recommendations for legacy NTSC television (ITU-R BT.601-5). The Rec709Luma space is based on the recommendations for HDTV (Rec. ITU-R BT.709-5) and is suitable for use with computer graphics, and for contemporary CRT displays. The GRAY colorspace currently selects the Rec601Luma colorspace by default for backwards compatibly reasons. This default may be re-considered in the future.
...

Here is an example of an image conversion, the output for PDFs would be similar.

Input image: input image

Output -monochrome: -monochrome

Output -colorspace GRAY (Rec601Luma) -colorspace GRAY

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