Possessing or distributing certain types of illegal content is a serious crime, even if your intention is to report it.
If the file is hosted on a specific website, cloud storage service (e.g., Google Drive, Mega, Dropbox), or social media platform, use their internal "Report Abuse" tools.
For broader internet-based crimes, you can submit a report at ic3.gov . Important Safety Steps
You can contact your local police department to provide information about the distribution of illegal content.