A few months ago I purchased several RPG books as pdf without any problems or delays. They weren't from Mongoose, but the webpage offered several Mongoose products, too, maybe you can find the F-series there in near future:
A problem with most pdf publishers is - as far as I see with my limited experience with this method of purchase - that the format is often not printer-friendly. Read in the product description whether they included a text only version (black / white) and the regular version (as the Darwin's World RPG does).
Hope that helps.