My PA employer's policy lists 8 paid holidays. If those holidays fall on a non work-day, such as Christmas on Sunday this year, are they required to still grant those days (i.e. observing the Christmas Day holiday on Monday)? Or is that at my employer's discretion?

There is no law requiring that employers provide any paid holidays, so there is no law specifying what to do if a holiday falls on a Sunday. It's entirely at the employer's discretion.