If you get stdio.h not found errors, ensure your "Folders/Extensions" path is pointing exactly to the folder containing the bin , include , and lib directories.
Once you have downloaded the zip file for the compiler, follow these steps to make it appear in your Keil dropdown menu: keil arm compiler version 5 download
You are maintaining with specific pragmas or inline assembly. If you get stdio
The safest and most reliable way to get the compiler is through the portal. Go to the Arm Compiler 5 Downloads page. You will typically need to sign in with your Arm account. keil arm compiler version 5 download