It's not possible to transfer/convert a pdf into a character sheet on Roll20, they are made in HTML/CSS & javascript here. So if a char sheet were to be made for Darwin's World, it would likely be best to start form the d20 Modern sheet if it seems close enough and then make changes. To be able to work on designing character sheets on need a Pro account. After comparing the sheet you provided and to the existing d20 Modern character sheet, I don't really see a difference, and suggest to see how well the d20 Modern sheet would work for you. What are the differences between them, do they vary in how things are calculated, or have many changes? If there exist some small-to-moderate changes that wouldn't be too hard to make, I could consider making an alternate version of the d20 Modern sheet. That is, if you can tell exactly what the differences between their mechanics are, I won't put time on making visual changes.