Some digital signatures are sometimes stolen or misused. To handle that situation each issuer of digital signatures lists a number of digital signatures that are revoked (Revocation lists - RCL's). So it is a good idea to check with the issuer that the current signature still is valid. You can verify the signature in at least two manners:

1) Verify during open:

You can e.g. setup Acrobat Reader to verify digital signatures when opening a PDF file by selecting 'Edit', 'Preferences' and 'Security'. Here you will see an option 'Verify signatures when the document is opened'. If you activate this option Acrobat will check the signatures when opening the PDF file. The downside is, that opening takes a bit longer.

2) Verify the signature manually:

Find the signature of the PDF document and right click on the 'Signed by XXXX'. Then select 'Validate Signature'.