New file/database in OmniFocus 2 for macOS
Apparently, there is no way to create a separate new file/database in Omni Focus 2. The default workflow is to use the pre-created database along-with the built-in backup feature.

You can create a new OmniFocus 2 database by following the steps as mentioned in the Omni Group thread, Create new database / start from scratch?


  Once you're satisfied with the state of your backups, you can remove your current OmniFocus database:
  
  
  Hold down the Command key and click on the small icon at the top center of your OmniFocus window.
  In the menu that pops up, select the second item (a folder named "OmniFocus"):
  Hold down the Command key and click on the small icon at the top center of your OmniFocus window.
  In the menu that pops up, select the second item (a folder named "OmniFocus"):
  
  
  
  
  
  This will open a new Finder window. Once this window opens, quit OmniFocus.
  In the Finder window, delete the file named OmniFocus.ofocus, as well as any files ending in lock.
  Close this Finder window and relaunch OmniFocus.
  
  
  You should be prompted to create a new database, either from a sync server (usually Omni Sync Server, or perhaps your own WebDAV server) or simply by setting up the default database locally. I usually start with the latter.
  
  If you do have your data synced to a server, and you want to get rid of that as well, set up a local database and tweak it to your heart's content. Once you've done that, you can set up sync with your existing sync server, then choose File / Replace Server Database... to erase any sync data on the server, putting your new empty database online in its place. Be careful - this means your only copies of your old database are the backups on your computer!