The problem: you have a PDF with some coloured elements (like some logo, separation lines, etc…) but you need a PDF in B/W.
The solution: you can print your file (with a B/W printer) and then scan it 🙂 or, using GPL Ghostscript, run this command…
gs -sDEVICE=pnggray -r150x150 -sOutputFile=output-%02d.png -- input.pdf
You can also try -sDEVICE=pngmono if you want a two colour version. With -r150x150 you choose the resolution, so -r300x300 will give you an image with 300DPI.