DocumentSignatureHelper::isOOo3_2_Signature() checks if the manifest
stream of the ZIP package is signed. A PDF file has no storage, so don't
do that check for that format.

With this, a valid PDF signature is shown as valid in the Digital
Signatures dialog as well.

Change-Id: I58c1cbd665b7c5894d1cfa193061a9370f76c8c4