I mostly use Preview.app to create simple pdf files (ever since I learned that you can add text to them with it). It suits my needs, and still would now... but if I try to select the "alpha" color (white box, red diagonal line) from the text-styling dialog, it does not let me set it to that.
I suspect that Preview.app simply does not have that capability.
I've also tried using inkscape. I import the image I want to use, put that on the lowest layer, add text above it, and then try to make it invisible. I've set its opacity to 0, I've tried setting its fill to "X", I even set A to 0 in its RGBA.
After saving each of these experiments to pdf, when I open in Preview, there is no selectable/invisible text.
Can either of these tools accomplish what I'm attempting to do? Is there another free tool (open source or otherwise) that can accomplish this?
Note: it's important that I be able to position the invisible text to match the background to some extent, and that it's selectable via mouse dragging.