Before digitally signing a pdf document, you must adjust the settings in Adobe Acrobat Reader as follows:
Open Adobe Acrobat Reader.
Select “Edit” at the top and then “Preferences…” at the bottom.
Select “Security (Enhanced)”. Uncheck “Enable Protected Mode at startup” and “Enable Enhanced Security”. Confirm by clicking on “OK”.
Now the program needs to be closed. Select “File” at the top and “Exit Application” at the bottom of the list.