What is the size of that pdf files?
Are you unable to download any pdf file or some specific pdf file?
Have you tried connecting to a different internet network and tried downloading the file again?
Thanks for the reply!
The PDF file sizes vary, it seems to happen more often with files over 1-2MB.
The reps are trying to download the files over their data connection (4G) and unfortunately they do not have any other feasible internet connections as they are on the road.
May I know the version fo iOS you are working with?
In iOS 9.x, Mail app displays the preview of the PDF there itself.
The screenshot that you attached has a file Label as 'E' however with PDF documents it comes with 'PDF' label.
Try to open the same file using the web. Let me know the result.