it seems that when I upload a pdf to the DAM, it does not generate any thumbnails? Is this correct. I would like to be able to generate jpg thumbnails in the same way as pictures do. Is this possible?
Yes, this is correct, no JPG preview is created for an uploaded PDF. If you click on the PDF icon in the preview pane, you will see a preview version in the annotation window. This would be a nice enhancement.
However, having not tried this, I don't know the side-effects or whether a JPG image would be properly rendered from a PDF file.
A better approach would be to extend the OfficeFileImporter to generate JPG preview files if the file type is PDF. This is more complex but would allow PDFs to still be treated/classifed as Office Documents.