If they are from the United States, it is advisable for you to know approximately their city and name. Next, you need full proof that you or your relative said they were under 16 years old. Then you will be able to use the screenshots as evidence against the PDF

Steam keeps the log dialogue for about a month, so you need to make sure to

• Steam Support
  In Steam support, be sure to have all the screenshots and write that the person who wrote and harassed you or your relative. It must be seen exactly how they did it. No blurs, if they sent you a ♥♥♥♥ pic, then you send a screenshot to support where it is visible in the dialog. Also, you must show where you wrote that you or your relative is a minor, which will help to ban this PDF, you can do without it, but this increases the chance of their ban and direct evidence against them.
  
  
• Cyber Tipline
  https://report.cybertip.org/reporting
  In this website, you can fill out an application against the PDF. You will need to fill out an application on your behalf or on behalf of your friend, relative and describe the whole situation there. You should save the screenshots you took earlier and send them later when you receive an email from the site. This site can help relay this information to the police. So don't forget to indicate what it was on Steam, provide their link, describe the whole situation, and write that you saved all the dialogues.
  
  
• FBI
  https://tips.fbi.gov/violation-category-selection
  Yes, you can officially file against the PDF in the FBI. You choose a reason and fill in everything you know about that person and write that you have screenshots. Later, you will receive an email from one of the detectives or their staff to clarify other aspects if they have accepted the application and are ready to work.
  
  
  Knowledge of other social media of PDF will help authorities find him so that he will suffer the punishment he deserves. Discord, Steam, YouTube, Spotify, Snapchat.
  
  Add them to the reports in the second or third method, it will really help to catch them by the police