Essentially all I did was to add a few new text boxes in another layer though, that's not really asking it to be a DTP program. Having said that the import is not exactly as the PDF original appears on screen as some fonts were substituted.

I've just pulled the PDF into Serif Pageplus X2, made a new layer and put in some text, saved and exported as new PDF and it works faultlessly. I'm not a rocket scientist, far from it, but what's the difference between adding adding some text in a new layer in XXP and doing the same in X2?