Can we download SSRS 2017 to use with SQL Server 2016?
You can download and install SSRS 2017, and use an existing SQL Server to store the Report Server database.  The databases can be on any supported version of SQL Server, as documented here.

The server running SSRS 2017 must, of course, be licensed per the licensing guide here.

Alternatively you can get this functionality (and more), with Power BI Report Server, which can be acquired via Power BI Premium, or run using SQL Server Enterprise Edition Core Licenses covered by active Software Assurance.