The backup PDF has been generated on your devices during signup and has never yet been sent to BTC.com, therefore the only one storing this secret document is the wallet user.
The backup PDF is required in case the password has been forgotten to the wallet.
Where could the backup PDF have been placed?
If you signed up on the web page you have Downloaded your backup PDF to your computer and perhaps even printed it out. Look in all your Download folders and think about which device you created you wallet on.
If you have signed up via the mobile BTC.com wallet app, check your email inbox if you have sent yourself the backup PDF.
If you signed up using an iOS device, you can check Dropbox, iCloud, iBooks as possible backup locations.
If you chose to store your backup PDF on the phone as well:
* On Android, check the app folder for the BTC.com wallet app on your filesystem
* On iOS, use 'iTunes Filesharing' to access your backup PDF through iTunes if available