Quite often, applications will use very thin lines, or even the value of 0, which in PDF means ”The thinnest possible line on the output device”. This might be fine for on-screen work, but when printed on a high resolution device, such as by a commercial printer, they may be too faint, or disappear altogether. The -thinlines option prevents this by changing all lines thinner than <minimal thickness> to the given thickness. For example:
cpdf -thinlines 0.2mm in.pdf [<range>] -o out.pdf
Thicken all lines less than 0.2mm to that value.