Not sure when this got broken, but if you add a PDF to a project, the original PDF is correctly copied to the output. But if you add a PDF and a THM as the representing image, the PDF is not copied to the output.

There isn't even a good workaround. If you add the PDF and make the album, the PDF is in the output. But if you then just add the THM and make the album again, the PDF is deleted from the output.

ETA: Zipped Minimal project

Did some testing. Working correctly in jAlbum 26.2, broken in jAlbum 27.

Looks like it's working again, both for PDF's and MP3's (as well as MP4's, which didn't get broken in the first place).

A better mechanism for representing images is really needed. A user should be able to provide one, even for a JPG. An extreme pano JPG might benefit from having a different image for its thumbnail. Back when I had MatrixPano, which dealt with equirectangular pano's, I did that in the skin. So, the pano might be the interior of an historic synagogue, but the thumbnail could be made from a normal JPG of the outside of the place.

When working with the project, it should be possible to see exactly what files are being used as the RI's. With the THM mechanism, the RI effectively vanishes - the only way even to know that one has been used is to look at the thumbnail, then open the object for editing.

But that will all require some extensive changes, both to the UI and to internal code. Maybe some rainy afternoon when you're bored.

I have updated to jAlbum v27.0.7 and Jupiter 51 and still have a problem with PDF's and THM , I cannot seem to add the THM files to the album either by dragging them or using the Add Option.

Remember, the THM file won't appear in the jAlbum Explore window - only the original, base PDF file will be shown. But if, after adding the THM, you hit F5 to refresh the display, the generic PDF thumbnail icon will change to the THM image you've just added.