If a product is referred to by an acronym such as pfl for personal fall limiters then when more than one would it be written as pfls without an apostrophe or with?

If you're writing the abbreviation like this (PFL), then you can just put PFLs to make it plural.

If you're writing the abbreviation like this (P.F.L.), then you can use P.F.L.'s to make it plural.

Both are correct.