SHARE Document WITH EXTERNAL USERS – SharePoint Online

Understand Share with external.  

https://support.office.com/en-us/article/Turn-external-sharing-on-or-off-for-SharePoint-Online-6288296a-b6b7-4ea4-b4ed-c297bf833e30

Tenant level changes:

https://support.office.com/en-us/article/Manage-external-sharing-for-your-SharePoint-Online-environment-C8A462EB-0723-4B0B-8D0A-70FEAFE4BE85

NOTE :

Need to deactivate below feature to check-out and check-in

Limited-access user permission lockdown mode

When this feature is enabled, permissions for users in the "limited access" permissions level (such as Anonymous Users) are reduced, preventing access to Application Pages.