When you (or your IT department) first setup your computer, you were prompted whether Adobe Trust Manager must automatically update. If you chose "No", Adobe is set not to update and the message does not appear again to prompt you. 

This mean that your Adobe Trust Manager is not up to date and you will need to manually update it in order to verify digital signatures. 

Please follow these steps:

1) Open up Adobe Acrobat Reader

2) Click on "Edit" (top menu), the choose "Preferences". 

3) Scroll down and next select "Trust Manager" in the list on the left hand side.

4) Click each "Update Now" buttons.Wait until updated.

5) Click "OK"

6) Close Acrobat Reader.

7) Click on the PDF file to open in Acrobat.

The digital signatures will validate.