Collaboration on a document may involve multiple people with varying roles. Some will be full contributors, fully enabled to edit the content. Others may be invited to review the content and offer editorial suggestions or comments. Individuals without such permissions may be able to simply view the document. The Share function lets you send a link to a document and specify what permissions are allowed for any recipient of the link (edit, review, or view).
Share a document for review
Select the Share button.
In the drop-down menu, select Share to open the Email link pane.
Select the people that you want to review the document, then select the Can review option.
When done specifying recipients/permissions select Send.
Note: You can access this option in a similar manner via Other settings in the Link settings dialog box.
Guidance for recipients
When you open a document that was shared with you for review, you are automatically placed in Review mode. In Review mode, you cannot edit but instead are allowed to add suggestions to the document in the form of comments or tracked changes. Document owners or other collaborators who have full edit permissions will then need to approve the incorporation of any of your suggested changes to the document.
File location considerations
This feature initially works only on documents stored in OneDrive for Business. For documents stored in a SharePoint library, the file permissions settings for that library will take precedence. Hence, those who already have read/write permissions for the document can open it in Editing mode – regardless of whether a Review mode link was shared with them.
Learn more here: Document modes in Word