In the below screenshot, you can see a whole list of pdf files that have been uploaded and have the little pdf icon next to the file. However, there is one file (HPCT_Anderson_RecommendedReading.pdf) that is a pdf but has a blank icon next to it.
Whenever I use the sidebar to embed a file into a content page, I get the preview icon if the file has the pdf icon, but I DO NOT get the preview icon if the file has a blank icon next to it. See below.
I don't get it. Why would some PDFs be previewable but one not be?