If you have backed up via certain tools, the backup files can be extracted easily and restored to a new device. But if you simply want to transfer files stored in PDF Reader to another device, you could try the following steps:

• Zip Files 

1. Switch to the Document tab for the file list
2. Select files and move them to one single folder
3. Tap “Edit” on the top right corner
4. Select the folder and choose “Zip” to compress the folder into a zip file
   

• Transfer to Desktop via iTunes USB

 You could transfer the zip file from your iOS device to PC/Mac via iTunes USB first and then restore it to a new device. Or you could choose to sync the zip file to another device through Kdan Cloud or other cloud storage services.

Note: For devices running iOS 12 and later, PDF Reader Premium and PDF Connoisseur as well as other 32-bit apps can not be opened. You can access your files located in these two apps via iTunes if you connect your device to PC/Mac with a USB cable.