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If I select image files in a Dired buffer and do C-t C-t, a nice little thumbnail appears. But if I try this with PDF files, nothing happens. I would expect to see a thumbnail of the first page of the PDF.
Is this normal? Is there an easy way to get the behavior I want? (I am running Emacs 24.3.1 on Ubuntu 14.04 if that matters.)
This is a feature of dired+ as far as I can tell. C-t C-t is bound to image-dired-dired-toggle-marked-thumbs, which first checks whether the file is an image or not with diredp-get-image-filename. This procedure runs image-file-name-regexp to generate a regular expression matching all image file names. Then it creates thumbnails with Imagemagick's convert (defined in image-dired-cmd-create-thumbnail-program).
You should be able to modify the behaviour of image-file-name-regexp by adding a regular expression for PDF files to the variable image-file-name-regexps. convert can convert PDF files to images, so you should get a thumbnail.
convert treat PDFs differently than other images, generating a numbered image for each page in the file. Unless there is a way to modify image-dired-cmd-create-thumbnail-program so that it would do something like convert file.pdf[0], but that may cause problems with regular images.