As per the recent communication from Microsoft, Microsoft Dynamics CRM SDK will not be available on Microsoft downloads. If you are working on Dynamics 365 v9, well you have to download every component which was available in the SDK individually.
Every tool will be available via NuGet Manager.
The name will be : Developer Guide not Dynamics CRM SDK.
For more information, visit the below blog : https://blogs.msdn.microsoft.com/crm/2017/11/01/whats-new-for-customer-engagement-developer-documentation-in-version-9-0
Here is the step by step information on how to download these tools using powershell : https://docs.microsoft.com/en-us/dynamics365/customer-engagement/developer/download-tools-nuget
i hope this helps.