The first thing you check after receiving of Invoice is how to convert yellow question symbol to Green tick symbol.
Follow below steps to validate your yellow mark to green tick.
Validation work with Acrobat reader/pdf applications.
Right Click on Signature area where Question mark appear.
Select Show signature properties as shown in below image
Click on Show Signer's Certificate as shown in below image.
Click on Trust shown in below image
Click on add to Trusted Certificates as shown in image above.
Click OK on Adobe dialogue box as shown in below image.
Check on below Options
Click on OK
Click on OK again ( This OK will be in bottom of window screen nearest to task bar check properly)
Now Your Document is ready with Green Tick