you may consider using some professional English editing service. Last time,
my boss asked me to use the one provided by elsevier. He told me that it
would save him lots of time and he only needs to focus on science of the
manuscript if there is no language errors. At the end, it only cost us USD
200-300. I also learned some writing skills from the professional editing.
PNAS is short, so it won't cost you too much money. come on, $200 from your
own pocket is not a big deal.