Hi David,
In your case, your PDF technically still opens in WebLink Document Viewer but since the pdf document itself has no Pages generated, the Document Viewer has no image to show you.
< This is actually WebLink Document Viewer.
You must first generate that pdf file's image. This can be done from the Client when you Import:

Or for existing documents, right-click on your pdf file and select "Generate Pages"

Generate Pages will create the Image component of your PDF. Then only, WebLink Document Viewer will have images for it to display.
(Note that WebLink Document Viewer can show document image or text; if your pdf or electronic document only has document text generated but no Pages, it will display the text in the Document Viewer)
Hope it helps.