Since we added iCloud Drive support in PDF Expert 6, the option to view documents via iTunes has changed. To make your files visible via iTunes, move them to the iTunes Files folder first:
Tap Settings > File Manager.
Enable iTunes Files.
Copy or Move your files to iTunes Files folder.
After this, you can easily save files to your computer via iTunes File Sharing.
Note: If you connect your device to a computer or laptop and move any document to the PDF Expert tab using iTunes, you will find the file in the iTunes Files folder in PDF Expert.