I uploaded to the Wikimedia Commons the PDF "File:John Stuart Mill, Considerations on Representative Government (1st ed, 1861).pdf" (https://commons.wikimedia.org/wiki/File:John_Stuart_Mill,_Considerations_on_Representative_Government_(1st_ed,_1861).pdf) which I had downloaded from Archive.org, but none of the pages will thumbnail properly. I tried purging the page to no avail.
I reported this problem at the Commons Village Pump, and another editor said that when he tried to view the thumbnail at https://upload.wikimedia.org/wikipedia/commons/thumb/7/70/John_Stuart_Mill%2C_Considerations_on_Representative_Government_%281st_ed%2C_1861%29.pdf/page1-76px-John_Stuart_Mill%2C_Considerations_on_Representative_Government_%281st_ed%2C_1861%29.pdf.jpg he encountered this error message:
"Error creating thumbnail: convert: no decode delegate for this image format `/tmp/magick-hg7YMuoz' @ error/constitute.c/ReadImage/532."
"convert: missing an image filename `/tmp/transform_b1c9d0271ec9-1.jpg' @ error/convert.c/ConvertImageCommand/3011."