When a PDF is imported it is first converted to a .PNG format. This format allows us to control transparency, but at the cost of using more memory.
Select the image then run the command writeworldfile from the command line. This will create a world file in the same folder as the image using the same format as the image. (PNG, JPG, TIFF, etc.)
If you import the image again (PNG not the pdf) and the world file is in the same folder, then the image is automatically placed and georeferenced.
A small treat for 2020.
Note that hidden commands such as this are typically used internally for debug and testing. They are not documented, tested in each release, or maintained. They may be removed from the product at any time. Usage is at your own risk.