Google allows me to make the file private but I am not sure that making it private has effect.
migrated from superuser.com Feb 28 at 19:02
I created a public folder. When I attempt to create a new document in that folder, Google Drive tells me:
This suggests that I can't have a private document in a shared folder.
But what if I created the document first, then made the folder public?
So I made the folder private again, and created a document in it.
Then I reverted the folder to public. Inspecting the document's sharing settings, it is now
Opening an incognito window in Chrome, pasting in the link to the document, gives me the Google login form.
I also created a new document in the folder, without altering its sharing settings. This is viewable in the incognito window.
Visiting the folder in the incognito window gives me a list containing only the public document. When logged in, I see both documents.
So yes, it does seem you can have a private document in a public folder. The folder's setting merely acts as a default for documents in that folder with no explicit setting.