By default company file is saved on Desktop.
If you are unable to locate kindly follow below steps.
If your company file is syncing via Dropbox, it will always be in
Dropbox > Apps > Book Keeper App folder.
If your company file is local,
1. open Book Keeper Windows
2. Take your mouse on Company name,
kindly note, just take mouse on company name, don't click anywhere.
3. It will show you company name and location in tooltip as shown below.
4. Open that location and there you will find company file.
5. Once you have located the company file, you can take backup, just copy this file to your Pendrive or email to yourself.