Page fees are usually paid out of the grant. Some science journals have, in the past, waived page charges for people not supported by grants or institutions, or who just couldn’t pay, sometimes at the cost of slower processing of the article.
“submission in some electronic formats qualifies for a publication charge discount (Physical Review Letters) or waiver (Physical Review A, B, C, D, E, Applied, Fluids). Acceptable formats for the discount or waiver are currently REVTeX (preferred), LaTeX, Harvmac, or MSWord”: https://journals.aps.org/pre/authors/submission-faq#discount