When you receive some PDF (not all), it is printed with black box on PDF, it is generally due to the PDF generator which includes image in a format not supported by internal system in Automatic Email Manager, 2 solutions:
1- Use external program to print pdf, instead of internal system in Automatic Email Manager: Click on top menu 'File' then 'Options', now in the section 'Email attachments' click on 'File type management', search for PDF extension, edit it and set to use external program.
Now make sure to have a PDF reader installed like Adobe Acrobat reader or Foxit reader.
This solution works fine only if you run Automatic Email Manager under a Windows session, if you run Automatic Email Manager in service mode (without any user logged) then it will not work.
2- The other solution is to change or update your PDF generator if it is on your side (and not on the customer side).