It sounds like you may have inadvertently applied a password to the pdf document and now you don't know what the password to unlock it is. Or maybe your pdf software applies a password.
I suggest you try re-downloading your certification document from Microsoft certificate website and start all over again. Make sure you are using pdf software (Adobe, Foxit, etc) that does not do something to your pdf file that you don't want, like apply
some security requiring a password to open again.
HTH,
Phil Streiff, MCDBA, MCITP, MCSA