You can transfer all files from App Store PDF Expert to Early Access update automatically. On launch of the Early Access PDF Expert the app will prompt to transfer data, just tap Yes on pop-up message to start.
You may choose to transfer files manually. You may want to do this is you would like to revise the files you store in the app. Before you install the Early Access edition, follow the steps below to upload the files from an app to a web storage. Just open PDF Expert and tap Edit in the top right corner of the app, tick folders and files you would like to move to new update and use Upload option to move them to any storage. (either it's Dropbox, Google Drive, Box, OneDrive or simply to your NAS drive)
NB: you may only do this if you didn't install the Early Access version.