Hi David!
To fix appearance of the mentioned Document Gallery instance I would recommend using DG feature for setting custom thumbnails.
Before doing that You’ll need to prepare a proper thumbnail. If You still want to have a cover (first page of the pdf file) to be as an icon, You should convert first page of the pdf to an image (png or jpg would be ok; You should set dimensions twice as big as the desired to have some freedom while editing), crop it (get rid of the unwanted white areas) and scale it to the desired size (so the image would fit thumbnail container; this step just lets You get the most efficient ratio size/quality). If You don’t have relevant software, You can find variety of free online tools, that would be of great help.
Then You should go to the Dashboard -> Settings -> Document Gallery -> Thumbnail Management of Your WordPress. Locate there row with the needed Document Gallery element and either Drag’n’Drop the prepared before thumbnail over to the defined row, or You can click on Select File button in the defined row and select the prepared thumbnail in the emerged dialog. (identical result can be achieved by using same uploading techniques on the Edit Media page of the required attachment, which can be found through the Media Library at Dashboard -> Media).
I hope this would work for You.
Please don’t hesitate to let us know if You run into any further issues.
