With Paid Memberships Pro, I can't appear to set any page to render as HTTPS unless it's one of those controlled by PMP. Since I'm trying to run Easy Digital Downloads as well, I'm finding that I can't render that checkout on HTTPS unless I deactivate Paid Memberships Pro.
Paid Memberships PRO v. 126.96.36.199
Easy Digital Downloads v. 188.8.131.52
Gateway enabled in PMP is Authorize.net
EDD has Paypal & Authorize.net