I have seen that some universities give postdoctoral fellows an official certificate (similar to the PhD diploma) after finishing their postdoctoral job. On the other hand, some of postdoctoral positions are just simple research (or even academic) jobs. I have even seen that some professors pay the postdoctoral fellows without contract (from their grants).
My question is: How much
certificate of postdoctoral studies is common? I mean Do future employers expect applicant to give them certificate of postdoctoral studies as well as PhD diploma?