In Vaults opened before 14 December 2020* Vault Admins can enable the Microsoft Office 365 viewer for external Guest roles in 6 easy steps:
Step 1: Navigate to Buy side;
Step 2: Navigate to Set permissions on the role you would like to enable the Office 365 viewer;
Step 3: Switch the toggle of the Microsoft Office viewer;
Step 4: Choose View or View & Edit permissions;
Step 5: Select OK;
Step 6: Select Save to save the changes in the permission screen.
*in Vaults opened after 14 December 2020 the Microsoft Office 365 viewer is enabled by default for Guest roles.