Hope you are doing well and sorry for the delay in response. As described sent pdf appear as blank.
I hope you would be able to fix it by now, if the issue still persists please try to logout of the application and reboot the device once and relogin to the application and check. Also, please try to update the application from play store (Android) or App store (iOS) and check
You may also try to send a document to your self for testing and see if its arriving blank.