We have recently implemented Nintex Sign powered by Adobe Sign. Everything works fine when the template is stored in Nintex Sign, it is successfully sent for signatures and uploaded to a library on our SharePoint server. 

We now have a need to send documents stored in SharePoint. However when I select SharePoint as the document location I receive an error Cannot Open File "FileName.pdf".

I have ensured all users have full control over the document in question. Our SharePoint is on a secure server, however the workflow is ran by a service account with Owner access to the site. As we are having no issues uploading the signed document to a library I am unsure why we are unable to open a file from the same library. Any ideas?