If one of your files is corrupt, or the PDF is incorrectly formed, we will report this by showing you an error in the file staging area of the assign screen.
You can delete the offending file and reupload, or, if the problem persists, try printing the file to PDF from your PDF viewer, and then re-uploading. In most instances, your PDF viewer will also have trouble with this file. You can also try to run a repair tool inside you PDF tool to fix the broken file. As a last resort, physically print the file and scan it to a PDF.