Why does filled PDF not display fields when viewed in Document Viewer, but does when file downloaded?

We have enabled the document viewer for case attachments, but have found an odd behavior that I am trying to track down the source of:

When a fillable PDF (with fields completed) is attached to a case and the agent clicks "view" under manage attachments, the completed fields are blank in the file viewer. However, if the agent clicks on the attachment and downloads, all fields are filled in as expected. 

To clarify, we are not using the PDF document functionality. This was a fillable PDF generated, completed, and saved outside of SN. It was uploaded manually to the case as documentation. 

It is not a huge issue, as we can just click the file to download, we just are getting stumped on the root cause of this one.

Any thoughts on what might be occurring?