PDF files that contain a single image are often a pain to work with, but it is simple to extract that image using pdfimages
.
First, run pdfimages with the list command to check the document is composed of a single image:
$ pdfimages -list file.pdf
page num type width height color comp bpc enc interp object ID x-ppi y-ppi size ratio
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1 0 image 3508 2480 rgb 3 8 jpeg no 8 0 300 300 409K 1.6%