Yes, non-members can be invited by sharing the team URL. They will see the team overview and will be prompted to 'register / sign in' in order to join and collaborate with your team.
Documents are shared via Google Drive. Make sure you have a Google Drive account (it's free!), upload your document there and set your sharing permissions on Google Drive. You can check the sharing permissions by using the 'lock' icon. If you want anyone within the team to view and edit your document, make sure you set 'link sharing' to 'anyone with the link'. If you want to keep the document as protected, you can either turn link sharing off and individually respond to people's requests to view, or you can add specific email addresses into the 'allowed' field.
Once you are happy with your document on Google Drive, you can return to your team. Within 'document sharing', use the button to add a document. If it's your first time, you will need to allow Google permission to create a link between your Google Drive and the platform. Don't worry, this link only allows YOU to view all your documents. You still have to choose which documents to share with team members.
Grant Google permission and then you should see a window with all your Google Drive documents. Select which document(s) you would like to share with your team. You can delete these document links from the team at any time.