After the file is deleted, it will be placed in the Recently Deleted folder; you will have up to 30 days to restore deleted files.
If you delete the file which was located in an iCloud folder of PDF Expert app, you can also restore it on iCloud webpage > iCloud settings > Restore files section. If the file was in a synced folder, please go to the cloud storage webpage (e.g. Dropbox webpage) > restore the file from the Trash section.
If you deleted the app itself, you can try restoring your device from the backup or restore specific files from the iTunes backup