While I’m not at all familiar with any of the etchables, a work around that should work is to use export as svg instead and then with a text editor edit the svg file and alter all the #00000 (black) fields to #555555 (grey to preserve current black) then change #ffffff (white/transparent) to #000000 (black) and finally change #555555 to #ffffff (preserved black to white/ transparent). It is possible there is a way to do this in Inkscape but I think this may be simpler. If you know how the colors are represented in the pdf file the same trick should work there, but I deal mostly with svg files not pdf (other than to read them 
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Peter