In DEVONthink Personal, select File > Export > Database Archive to a data safe location (ie. NOT a cloud-synced folder. We suggest a folder for your databases in your Home directory) . This creates a ZIPPED version of the database.
In the Finder, locate the ZIP and double-click it to decompress it. You can now delete the ZIP file.
Add .dtBase2 to the end of the name of the decompressed folder (telling it to add the extension when it warns you).
Double-click it to open it in DEVONthink Pro / Pro Office.