If you want to post your Google+ stream directly from Google Drive, then make sure the file's sharing settings are listed as 'Public on the web' or 'Anyone with the link'. If you see that your file is private, then your family and friends on Google+ will not be able to access it. Once you shared a file, a thumbnail preview of it will appear in your Google+ stream.
Following are the steps to Post to your Google+ Stream Directly from Google Drive:
- Open the file which you want to share.
- Copy the file's URL from your web browser's address bar.
- Go to https://drive.google.com & sign in to your account.
- Click "Share" button located at the top right corner of the window.
- Paste the URL of file into the text box where it says "Share what's new...".
- Type the name of individual people or select a circle that you want to share your post with.
- Click "Share" button.
- This way you can post your Google+ stream directly from Google Drive.