How are you trying to move it to Drive? With the Share command?
How big is the file?
Also, in your Files app, what happens if you open the PDF file (which should open in Drive's PDF viewer), and then tap the Add to Drive icon at the top?
Try clearing the Google Drive app cache/data in Settings>Apps, then let it resync and try again.
Clear the app's data as well. It'll resync immediately upon reopening Drive.
Also, are you certain that you have plenty of free space left in your Drive account?
Did you already try clearing the Google Drive app data as well as the cache, as I previously suggested?