We are able to view thumbnails for assets that are images, but are unable to view thumbnails for PSDs, PDFs, and PPTs. Should this be the case? If not, any suggestions for a remedy would be greatly appreciated.
That is not the case. We generate renditions for many doc and other formats. Review - https://docs.adobe.com/docs/en/aem/6-2/administer/content/assets/assets-formats.html
Which version it is for?
Thumbnails are only applicable for images. Non-image assets like PDFs do not have thumbnails.