YG, I haven’t seen Low Roller’s document (BGG requires a person to register in order to download files), so I can’t really comment on it – but here’s an idea that you might want to consider for your own rulebook project, since it may save you a lot of work and may perhaps also be safer from a legal point of view.
I imagine that your project to rewrite the entire Global 1940 ruleset as a combined document will require you, as a first step, to create an outline of the document that you want to create – basically, a plan for the structure of the whole document, arranged in whatever way you think will be best from the point of view of clarity and ease of use. I also imagine that, as another preparatory step, you’ll need to make an inventory of all the parts of the Europe and Pacific 1940 rulebooks that you wll want to use, with page references to those parts of the two rulebooks. One way to make that inventory would be to plug the page references into your structured outline, to give yourself a complete plan of where in the rulebooks you’ll need to go to write each section of your projected document. (This may not necessarily be the way you were planning to prepare the project, but for the sake of argument I’ll assume that this was going to be your methodology.)
Anyway, what occured to me is this. Having prepared a detailed outline of the type I’ve described, and having inserted into the outline all the required page references to the Europe and Pacific 1940 rulebooks, you might not find it necessary to re-write anything at all. The document I’ve described would serve as a kind of sophisticated index – a conceptual one, not an alphabetical one – to the Europe and Pacific 1940 rulebooks that treats them in a combined way and that provides access to their content according to whatever logical structure you think would be most convenient for use by players. The players would still have to go from your document to the rulebook page to which you referred them, of course, but the point is that you wouldn’t have to spend any time actually rewriting the two rulebooks, which would save you an awful lot of work. Furthermore, since you’d neither copying-and-pasting the rulebooks (which would be questionable from a legal point of view) nor re-phrasing the actual content of the rulebooks (basically trying to get around potential copyright problems by putting the rulebooks into your own language), you might be on safer legal ground. Essentially, you’d be providing people with a structured and easy-to-use way of accessing official rules, rather than either reproducing or rephrasing the rules.