Windows installer of roughly 90 MB that the name identifies as the Microsoft .NET Framework 3.5 package. Older programmer utilities, EC tools and service software often depend on this runtime and refuse to launch without it. On newer Windows it can also be enabled through optional features. Keeping an offline copy helps on workshop machines without internet. Verify the digital signature before installing, as no exact build number is given.
The button opens the matching file on RM Schematics, or a source page if it is not catalogued there yet. The original attachment is from @opendatasheet. Enginerys does not host this file.