You can check whether your file is in Bin. For this, click the Bin icon in the bottom right corner. If the needed file is here, simply drag & drop it out of the Bin.
If you use Time Machine to back up your Mac, you can recover your files as described in this article. Please note that due to the design specifics of the macOS system, all files you've ever opened in PDF Expert are saved locally on your device, not in the PDF Expert app, and can be modified via the Finder app.
After the file is deleted from PDF Expert for iOS, 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 the PDF Expert app, you can also restore it on the iCloud webpage > iCloud settings > Restore files section. If the file was in a synced folder, please check the 'Recently Deleted' folder within your PDF Expert app.
If you deleted the app itself, you can try restoring your device from the backup or restore specific files from the iTunes backup using iMazing software on Mac/ PC (or any other software which allows browsing the backup) without restoring the whole device from the backup.