I think it's safe to assume it's legit and acceptable. This is from the Dartmouth Pre-Health Advising website:
PrivateFolio is the dossier system we use for supporting letter writers to upload their letters of recommendation. It is also a confidential and permanent home for your letters. PLEASE NOTE: All new medical school applicants must use PrivateFolio. If you have an existing Interfolio account and letters/documents already uploaded, you may continue to use Interfolio.