Business users have been looking for the feature of providing comments for Reports in SSRS. Until SSRS 2017 this has been mitigated by embedding the SSRS report in a .NET application and provide ability to input comments on a .NET UI and saving them.
With SSRS 2017, now comments can be directly provided for reports (Paginated and Mobile) using the SSRS Portal.
Comments can be added at Report level and these comments are stored in the Comments table in the ReportServer Database.
You can also add attachments as part of your comments and these attachments will be linked in the Catalog table in the ReportServer Database.
Security can be applied through the SSRS roles for comments also.