Steam accounts can be created if you are 13 or older.

What is your budget for a PC?

Steam families can give an adult account full control over a child account and what they can see or do on steam. You can read more here:
https://psteamhelp.yuanyoumao.com/en/faqs/view/054C-3167-DD7F-49D4

There are no portable document formats (.pdf) on Steam.

引用自 me<3：

i was gonna make my 14 year old cousin a steam account for his computer that im giving him for his bday also should i even allow him on steam or is there parental controls that i can help his parents set it up. if you can help i be happy yay :) also if somone can think of a decent pre made pc that would be nice ok ba bye

There is Family View that can be set on an account with a PIN to control what that account can do. There is Steam Families that one can join as a Child account and through that an Adult can restrict where and what the Child can do on Steam.

As for the query in the subject, they are everywhere. In all walks of life, in every community, there is an equal distribution of the personality spectrum. Do not fear the PDFs. Teach your cousin how to spot them and learn how to be vigilant when confronted by one.