Is it possible to convert a colored rgb photo into a true 8 bit gray scale image in Designer Pro 7 for output to professional black & white printing? (.pdf files are required)

I created a file I though would be gray scale in Designer Pro 7 and it turned out to be a desaturated rgb image instead. When it was reproduced, the black type on the page was no longer 100% black, but a lighter shade of gray instead. My supplier insists on files that are completely ready to print and will not attempt to make minor corrections such as this.

I do not have programs such as Illustrator, Photoshop, or even Acrobat. (Most of them would not run on my old computer even if I did have them.)

Thank you for your response.