Run through the steps you are taking, from the point of downloading from DropBox to the point where you see the error. Tell us all the things you do and the commands you are using to do them. I can't tell for sure if the files were unzipped, for example, or if perhaps you've tried to open instead of place them.
Does the file have any odd characters in the name, perhaps a back slash?
Try selecting Show Import options when placing the file.
Also, how sure are you that the file is a JPEG, as opposed to some other image format wth a JPEG extension? Open it in Preview.app and hit Cmd-I?
No - but it seems to be resolved by my saving the jpg files in a folder
higher up the food chain, outside of the folder where the original pdfs are
stored. So it seems to be working now
Thanks for getting back