Hi,
Thank you very much for the proper details, it helped us a lot.
The issue was, that the PDF file mime type was simply not entered on the index table options panel. I have selected it for you, and finished the indexing as well.
I have also enabled the index table engine on the search instance options, now you should be seeing the PDF attachments in the results list as well.
For all the details of PDF search, you can check the PDF content indexing documentation.
I hope this helps!