Even modern antivirus can miss a new or polymorphic threat. Upload the file to (virustotal.com). This service scans your file with over 60 different antivirus engines.
: Find .NET Framework 3.5 (includes .NET 2.0 and 3.0) in the list. dotnetfx 365.exe
These are legitimate, essential system files developed by Microsoft. The "fx" stands for "framework," and the numbers usually denote the version (e.g., 35 for version 3.5). Even modern antivirus can miss a new or polymorphic threat